Results Class for tukeyhsd, added confidence interval plotting functionality 

In response to issue #789, created a results class `TukeyHSDResults` that more concisely contains the output of a tukey honesty significant difference test from the `MultiComparison` class. This class now contains all the raw output that MultiComparison.tukeyhsd() used to, plus a couple extras. The Hochberg interval plotting i discussed is not contained within this new class, such that the expected workflow would be something like:

```python
mc = MultiComparison(data, groups)
results = mc.tukeyhsd()
print results.results_table #the summary table we were used to before
results.compute_plot_intervals(<name>)
```

The current workflow is also compatible with the `pairwise_tukey` function.