Dynamic loading of slf4j binding does not work

See https://github.com/culturegraph/metafacture-runner/issues/2

This commit adds the log4j binding for slf4j to the class path.

Our attempts to load the slf4j binding dynamically from the
`provided-libs` dir does not work as expected. slf4j does not consider
the context class loader for loading its binding but relies on the class
loader which was used to load the classes of the slf4j api. Until a
solution is found for this problem, the binding need to be placed on the
class pathslf4j does not consider
the context class loader for loading its binding but relies on the class
loader which was used to load the classes of the slf4j api. Until a
solution is found for this problem, the binding need to be placed on the
class path.

Author: cboehme

pom.xml

@@ -222,27 +222,34 @@
 			<version>2.0.0-SNAPSHOT</version>
 		</dependency>
 
-		<!-- The following dependencies are placed in the "provided"
-		     scope to prevent them from being included in the class
-		     path but still have them available for building the
-		     distribution assembly. 
-		     The reason for this is that appropriate implementations 
-		     for slf4j and jndi depend on the runtime environment and 
-		     should be chosen by the user of the package and not the 
-		     package implementer. However, since we are not also 
-		     building a distribution package which should include 
-		     everything that is required for metafacture-runner/Flux 
-		     to work out of the box. By including the dependencies 
-		     in the assembly without putting them on the class path, 
-		     the user can replace them easily with other 
-		     implementations if necessary. -->
+		<!-- In a perfect world the slf4j binding would be a
+		     provided dependency so that the user can replace
+		     it easily with a different binding. However, slf4j
+		     does not use the context class loader for loading
+		     its binding but the same class loader which was 
+		     used to load the classes of the slf4j-api. Until
+		     a solution is found for this problem, the binding
+		     need to be placed on the class path. -->
 		<dependency>
 			<groupId>org.slf4j</groupId>
 			<artifactId>slf4j-log4j12</artifactId>
 			<version>1.7.6</version>
-			<scope>provided</scope>
 		</dependency>
 
+		<!-- The following dependencies are placed in the "provided"
+		     scope to prevent them from being included in the class
+		     path but still have them available for building the
+		     distribution assembly. 
+		     The reason for this is that an appropriate implementation 
+		     for jndi depends on the runtime environment and 
+		     should be chosen by the user of the package and not the 
+		     package implementer. However, we also attempt to build
+		     a distribution package which should include everything 
+		     that is required for metafacture-runner/Flux to work out 
+		     of the box. By including the dependencies in the assembly
+		     without putting them on the class path, the user can 
+		     replace them easily with other implementations if 
+		     necessary. -->
 		<dependency>
 			<groupId>simple-jndi</groupId>
 			<artifactId>simple-jndi</artifactId>