Test and update examples to work with ARC files of the 2008 - 2012 crawls

[sebastian-nagel]

warcio is able to read ARC files as well, so it should be possible to run all examples designed to work on WARC files also on ARC files from the 2008 - 2012 crawls.

• needs to be tested whether the WARC examples can be run successfully on ARC files, minor modifications may be necessary
• could extend some WAT/WET examples so that they can be used also on WARC/ARC (cf. server_count.py which may use WARC or WAT)