Mac Editbox Text Font and Placeholder features

Hi,

For the most part, I did not alter any existing code. What I did mainly is focus on implementing missing features in the Mac implementation of `EditBox`.

The implementation focuses on introducing missing functionality related to `EditBox`'s text and placeholder. The Mac implementation was missing setting the font of the text, as well as the font and color of the placeholder.

The solution for the text was trivial, which was by setting the `font` property directly.

For the placeholder, I had to retain a dictionary of attributes and use `NSAttributedString` for the placeholder in order to control its properties.

Finally, I have slightly updated the tests, and ran them on Mac OS 10.9, and they seem to be working.

**Note**: You can probably see that all the changes I made are to code that is compiled for Mac OS only. Other platforms are not effected by these changes.

Best,
Maz