New attribute to require the tag to match a regex, before is added.

This attribute uses a regular expression to only add tags that correctly
match it.
Now, once you press backspace to delete a tag, returns to the input.

This is useful to edit recently added tags.
Replaced the attribute class for classes.

Change done to make it apply the class only to the inner div, instead of
tag-input AND the inner div.
Changed test, so the attribute "classes" pass it
Test for the pattern attribute
Test for moving the last tag to the input field if is deleted with double backspace
Merged pull request from Christopher Luna
Changed option class to ngClass
Changed option pattern to allowedTagsPattern
Changed option allowedChars to allowedCharsPattern
Added option enableEditingLastTag
Updated the README file