Trying out an alternative plugin scheme for pybtex.
Update for new name of method.
Use the final plugin registration API which is (just) now merged into pybtex.
Also update documentation.
Skip custom style test for now.
The babel package no longer supports Python 3.2, so omit it.
Also remove from setup.
Merge pull request #58 from mcmtroffaes/feature/simplified_custom_plugins

Simplify custom plugins using the new pybtex API.