V0.3.0

In this version removing non digits in a string is much faster thanks to suggests from people in elixir forum.

And there are two slight changes in API

1. Exceptions in remove non digits are now accepting char arrays instead of array of char arrays (which was a mistake of mine) e.g.: ~c[123] instead of ['1', '2', '3']
2. function is_digit_only? is changes to digit_only? (although is_digit_only? exists just for a little backward compatibility.