Implemented documentation.CorpusDocumenter.

Reorganized the documenters class hierarchy to inherit from a new Documenter
class, housing some basic ReST-generating logic, and other practicalities.


git-svn-id: http://music21.googlecode.com/svn/trunk@3490 5c16147e-a200-11de-b0cc-ab0ad15e60d0
Improved formatting of corpus reference in documentation/.

Source links for virtual now works appear on their own line, aiding 
readability.


git-svn-id: http://music21.googlecode.com/svn/trunk@3491 5c16147e-a200-11de-b0cc-ab0ad15e60d0