Added Date and Time Picker

This commit includes two new types for option-tree. Date Picker only
calendar and Date Time Picker with Calendar and Time. Both types were
tested with wp3.8 in twentyfourteen.

![456991f6-7fff-11e3-801d-720740665e7e](https://f.cloud.github.com/assets/2385217/1947649/55b01682-8060-11e3-914b-fc3ea90be0d2.png)