Add keystrokes per day plot 📈📊✨

This pull request adds 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:

### Scatter Plot Linear Interpolation

![screen shot 2014-12-05 at 12 20 21 am](https://cloud.githubusercontent.com/assets/375307/5326517/7cb6093a-7cdb-11e4-97d1-c0592790683f.png)

### Scatter Plot Histogram Interpolation
![screen shot 2014-12-06 at 3 07 10 am](https://cloud.githubusercontent.com/assets/375307/5327038/ab8f928e-7cf5-11e4-9a6f-255a8dae8dbe.png)

### Scatter Plot Curve Interpolation (no markers)
![screen shot 2014-12-06 at 3 09 30 am](https://cloud.githubusercontent.com/assets/375307/5327039/b08c821a-7cf5-11e4-91c6-c98c73164609.png)

### Scatter Plot Curve Interpolation (with markers)
![screen shot 2014-12-06 at 3 10 07 am](https://cloud.githubusercontent.com/assets/375307/5327041/b5e2751c-7cf5-11e4-9bf1-8a9a23c9320c.png)

### Bar Chart
![screen shot 2014-12-05 at 12 17 15 am](https://cloud.githubusercontent.com/assets/375307/5326516/7c73690e-7cdb-11e4-9582-2b149d6fa035.png)