Improve border-radius for better visual of selected range.

This pull request make better use of border radius to (IMO) help the visual flow between the two calendars and makes the start/end date more clear. Creating more of a bookend feel.

## Before
![screen shot 2014-07-09 at 10 28 57 am](https://cloud.githubusercontent.com/assets/945202/3526740/834ac3c6-077e-11e4-8814-f2ed43a8792d.png)


## After
![screen shot 2014-07-09 at 10 28 08 am](https://cloud.githubusercontent.com/assets/945202/3526743/8ee74164-077e-11e4-85ef-06f48653afb2.png)