Use custom metrics method to avoid displaying problems

This pull request solves the bug #18 by using a custom measuring method. This method is implemented as a category of NSString.

This is inpired by this [post](http://stackoverflow.com/questions/3374591/ctframesettersuggestframesizewithconstraints-sometimes-returns-incorrect-size/10019378#10019378) and  [post](http://stackoverflow.com/questions/5511830/how-does-line-spacing-work-in-core-text-and-why-is-it-different-from-nslayoutm/5635981#563598)