Bugfix for the issue #109: Fails to properly parse NOSE_COVER_PACKAGE environment variable.
Now the following line works correctly:

os.environ['NOSE_COVER_PACKAGE'] = 'package1,package2,package3'
Add a test for #109: Fails to properly parse NOSE_COVER_PACKAGE.

This also provides a place to do more testing of the coverage plugin.
Merge pull request #520 from Bahus/patch-1

Bugfix for the issue #109: Fails to properly parse NOSE_COVER_PACKAGE
environment variable.  Also added a test.

Note: I did not merge the other patch on this branch (ffc3701) because
I'm not sure it's the right fix.