Add plots for modalities

```
import flotilla
chr22 = flotilla.embark('http://sauron.ucsd.edu/flotilla_projects/neural_diff_chr22/datapackage.json')
chr22.splicing.plot_modalities()
```
Should result in:

![image](https://cloud.githubusercontent.com/assets/806256/3476990/ea15057e-030c-11e4-89b8-da96685086dc.png)

Still need:

- [ ] Bootstrap resampling of each splicing event to get a range or confidence interval or something (notice the red splicing event near the green middle ones)
- [ ] Splicing movies
- [ ] MOdalities bar plots