Change Kang::Data#match_group_count method to return 0 instead of nil when
there are no matches. Corresponding changes for all callers of the methods.
Add gui checkbox to turn on/off Regexp::EXTENDED.
Bump version of GTK and related libraries to new 2.1 branches. This adds
support for Ruby 2.1 but keeps support for 1.9 and 2.0.
Added instruction under OS X to fix libffi compile issue.