Add keystrokes per day plot

This pull request add a "keystrokes per day plot" to the main screen. In summary the changes contained are:

- Add `getKeystrokesPerDay` to `YVBDataManager`.
- Add CorePlot.
- Update FMDB.

I ended up settling for a scatter plot, but I also tried a bar chart, see here:

![screen shot 2014-12-05 at 12 20 21 am](https://cloud.githubusercontent.com/assets/375307/5326517/7cb6093a-7cdb-11e4-97d1-c0592790683f.png)

![screen shot 2014-12-05 at 12 17 15 am](https://cloud.githubusercontent.com/assets/375307/5326516/7c73690e-7cdb-11e4-9582-2b149d6fa035.png)