added kerning support

I've added kerning support for the library. 

I've also added a new function:
BitmapString:applyKerning(flag)   flag is either true or false
This allows the user to disable kerning for a specific string if needed (think timers etc)
By default kerning is enabled.

In my testing it works well. 
Not all fonts include kerning info. If no kerning info is found, the kerning will default to 0.