Add outline and shadow rendering to gdx-freetype

The current rendering styles of gdx-freetype are somewhat limited, requiring you to revert to external tools if you want any more fancy effects, such as outlines or shadows. Without shadow, text is really hard to read in any normal game.

To address this issue, this request adds a couple of new font parameters (documented in code):
- color: Foreground color; required as multi-colored fonts cannot be colored on rendering by GL. Defaults to white like before
- borderWidth / borderColor: Outline, defaults to off. Note: large border widths may lead to overlap, but this should be way beyond the "looks good" zone
- borderStraight: Line ending style for outline (curved or straight)
- shadowOffset / shadowColor: Shadow, defaults to off

Example rendering:
![gdx-freetype](https://cloud.githubusercontent.com/assets/5091018/3770809/022612fa-18ef-11e4-8f64-09bfb8e29044.png)

While implementing, I found that the current freetype JNI interface was a little weird, so I did some refactoring (first commit).

Only tested on Win64.