Add pandas 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).

Also added IPython Notebook integration for the stats results class (nows displays as an HTML table). Fixes #203.

This pull request makes pandas an skbio dependency.