Features
* Added statsmodels inference [22](https://github.com/biocore/gneiss/pull/22)
* Added support for ordinary least squares regression [33](https://github.com/biocore/gneiss/pull/33)
* Added support for linear mixed effects models [38](https://github.com/biocore/gneiss/pull/38)
* Added RegressionResults object to summarize statistics from statistical analyses
* Adding in a niche sorting algorithm `gneiss.sort.niche_sort` that can generate a band table given a gradient [16](https://github.com/biocore/gneiss/pull/16)
* Adding in utility functions for handing feature tables, metadata, and trees. [12](https://github.com/biocore/gneiss/pull/12)
* Adding GPL license.
Bug fixes