Add custom font support

Now, you can simply use:
``` objective-c 
[_tabbar setTextFont:[UIFont fontWithName:@"Chalkduster" size:14]];
```
to achieve this result:

![Screen Shot 2013-04-09 at 18 58 40](https://f.cloud.github.com/assets/502063/357798/9c0d3806-a12e-11e2-9125-a22697cab184.png)