Added EggDiscoveryLoader Plugin for discovering tests within Python Eggs...

Another attempt at Tests Discovery within egg files.
This fixes https://code.google.com/p/python-nose/issues/detail?id=78 thanks to a plugin.

Usage is not ideal as the default discovery plugin must be excluded as it fails if it sees something that's not a proper directory.

Usage: 
nose2 --plugin=nose2.plugins.loader.eggdiscovery --exclude-plugin=nose2.plugins.loader.discovery pkg_within_egg

Limitations:
nose2 --plugin=nose2.plugins.loader.eggdiscovery --exclude-plugin=nose2.plugins.loader.discovery path_to_egg_file.egg
does not work as one other default plugin is considering this to be a package name and stopping the discovery too early for the eggdiscovery plugin to handle it properly...