Make all ids strings instead of ints
Fix tests for string IDs
Work with IDs that aren't contiguous
Fix pertentage tests to reflect the new reality
Ensure all uses of groups are symbols
Add tests for passing non-symbols for group names
Merge pull request #20 from eric/make-ids-strings

Make all ids strings instead of ints
bump to 2.0.0c
Removes double occurence of features in feature list.
Merge pull request #23 from ayrton/fix-double-occurences-in-feature-list

Removes double occurence of features in feature list.