Extended styling suppport

Thanks for updating the library for Android 5 support. I've added a few extra options to your version, for my own use really but thought you might like to merge them in.

* ColorStateList support for text colour
* Change text style (normal, bold, italic)
* Change text alpha

All defaults should be the same as before.
A problem using `ColorStateList` is that `TypedArray. getColorStateList(index)` doesn't offer a second parameter `default` so we need to check for a null value if we don't want to end up with default black text instead of the colorPrimary. Something like:

```java
    if(colorStateList!=null){
        tabTextColor = colorStateList;
    }
```