Fix a crash and handling of adjusting font size

This request includes 3 fixes, all around adjusting font size:
* Adjusting font size doesn't work with numberOfLines > 1
    When setting number of lines to be greater then 1, and trying to constraint it to specific width, TTTAttributedLabel tries to calculate the ratio in which to 
    adjust the font size. I think the comparsion is wrong for multiple lines.    
* When label font size is scaled, app will crash as NSAttributedStringByScalingFontSize expect font attribute to be UIFont, thought it can be CTFont.
* When label font size is scaled, other attributed except font are being removed.

I was't sure if you prefer separate pull requests, so I hope it's ok that I aggregated them into one.