Added .ttf to .fnt generator to build-tools generators.

Didn't know where to exactly place this "extension" to I added it to the build-tools/generators.

Since I had to use it in order to display a decent Mockup font withing the Atlass .png file, I figured out you could need it too.

This converts .ttf filed to a .fnt file that can be packed within other texture Atlas (skin) or just to avoid generating the font each time the application is opened. 
Usage:

```java
// create .fnt from .ttf
new Ttf2FntGenerator().createFont( <.ttf fileHandle>,
						fontName,
						FONT_SIZE,
						<outputDir fileHandle>);
// Optionally you could specify the width and height of the output image via #setPageSize(int)
```

I think that if you use this you wouldn't need ExtendedSkin.java not ExtendedSkinLoader.java anymore since you would only have to load the .fnt file generated.