Add DataFrame support to distance-based stats.

Can now supply a grouping vector (as before) or a pandas.DataFrame and a column name to denote the grouping of objects. This makes interaction with the distance-based statistical methods more convenient in some cases (e.g., if you have a QIIME mapping file loaded as a data frame).
Add HTML table repr to results class.

For integration with IPython Notebook. Fixes #203.
add docstring for _df_to_vector

Thanks @josenavas!
Remove unnecessary variable assignments from tests

Thanks @josenavas!
Rewrite _df_to_vector to make better use of pandas
Use df.loc for more efficient lookups

Thanks @Jorge-C!
Merge pull request #306 from ElBrogrammer/dbs-pandas

Add pandas DataFrame support to distance-based stats