Added AbsoluteSourceFilter for changing relative src to Fully Qualified

This is useful if an image is root relative but should really be going through
a cdn, or if the content for the page assumes the host is known i.e. scraped
webpages and some RSS feeds.
Made AbsoluteSourceFilterTest exceptions include their filter class name.

This should help identify what contexts are required for the filter.
Test have also been updated to make sure exceptions are raised when
required contexts are missing and that the exception message includes
the name of the class where the required context is missing.
Merge remote-tracking branch 'benubois/relative'

Conflicts:
	README.md
Merge branch 'master' of https://github.com/jch/html-pipeline