Add Selection Modes to date picker.

There's a new option for the date picker called `selectionMode`. It's value is one of the following date picker global variables:

```
$.fn.datepicker.DPGlobal.selectionModes.DAY
$.fn.datepicker.DPGlobal.selectionModes.WEEK
$.fn.datepicker.DPGlobal.selectionModes.MONTH
$.fn.datepicker.DPGlobal.selectionModes.QUARTER
$.fn.datepicker.DPGlobal.selectionModes.HALF_YEAR
$.fn.datepicker.DPGlobal.selectionModes.YEAR
```

This specifies the type of selection you want to have on the calendar. If you choose `WEEK`, for instance, then it choses the nearest beginning of the week (respecting the weekStart option). It also impacts how the calendar is displayed, to reinforce what options you're actually selecting.

/cc: @peaches