Center DatePicker on iPad

Center the DatePicker in the available width on the iPad.  "Custom" pickers already take up the full width so no change is necessary there.

### Before
![before](https://cloud.githubusercontent.com/assets/90451/4485162/191f7c14-49c6-11e4-95b5-d5c1af199886.png)

### After
![after](https://cloud.githubusercontent.com/assets/90451/4485166/2081a608-49c6-11e4-9684-01b8284b55f2.png)