Read the existing MANIFEST.in file for files to ignore.

The existing MANIFEST.in can have been setup to ignore some files or complete directories. check-manifest should not complain that those are in version control but not in the sdist.

If MANIFEST.in exists, we read it and add relevant entries to the IGNORE definition.  To properly support the `exclude` directive I needed to add an IGNORE_REGEXPS definition.

Tests pass on the two supported pythons that I have: 2.7 and 3.3.