Add remove_class() method in Helpers class.

This method removes the given value to an array.
Deemphasize's button shouldn't have 'btn-default' CSS class.

Regression introduces in commit 2b09f06 and referenced in issue #184
Add one more assert for deemphasize's button test to ensure that 'btn-default' CSS class is removed.
Typo in a Button::deemphasize() comment.
Merge pull request #187 from tortuetorche/develop

[Bootstrap3] Deemphasize's button shouldn't have a 'btn-default' CSS class