added possibility to modify duration without using builder

(**Don't merge now!**)

I added the possibility to use 
`public static Crouton makeText(Activity activity, CharSequence text, Style style, int durationInMilliseconds)` (other methods also provided) instead of the 
`public static Crouton makeText(Activity activity, CharSequence text, Style style)`.

*There is only one, problem, which concerns me:*
I had to remove the `final` declerator from `durationInMilliseconds`, I don't know a solution **now**, but I guess, I'll find it in the near future. If you have the clue, just tell me and I'll implement it.