Added min/max-size options

- Adds the ability to define `min-size` and `max-size` attribute for ui-layout child elements. 
- Also adds a `dividerSize` option that allows you to adjust the size of the divider without having to edit the default CSS.

Unit tests for initial child element sizes included. Missing unit tests for attempting to move the splitbar outside of the min/max bounds. (Not sure how to simulate the splitbar drag/drop with Jasmine unit tests)

Resolves #3