Add NucleotideSequence.nondegenerates()

Also added @total_ordering and __lt__ to allow native sorting of BiologicalSequences.

Still needs more testing and docstrings.
Remove native sequence sorting.

It works to specify `key` in `sorted` calls, no need to add complexity to the existing sequence classes at this point.
Remove unused import
Add more tests, docstrings, gap character support
Performance improvements for ~2x speedup.

Is now slightly faster than version in PyCogent.
Use generator comprehension, add comment about try/except

Thanks for the suggestions @wasade!
add @wasade's suggested comment to try/except
add comment about other performance-optimized code
union non-expandable chars outside loop

Thanks @wasade!
Merge pull request #224 from ElBrogrammer/expand-degens

Add method to expand degeneracies