Add support for installing LibraryProjects

[wildabeast]

example of a plugin with a LibraryProject:
https://github.com/phonegap/phonegap-plugins/tree/master/Android/BarcodeScanner

spec:

    <android>
        <library-project src="LibraryProject" />
    </android>

expected result from pluginstall on above spec:

1. copies src/android/LibraryProject to project_dir/LibraryProject
2. adds the following line to project.properties:
   android.library.reference.1=./LibraryProject

[wildabeast]

Cross-posted in cordova-plugin-spec alunny/cordova-plugin-spec#8