Appearance protocol support

Added ability to specify the text / placeholder colors using
UIAppearance.
Fixed update of the color of the placeholder.

Bug occurs when you set the color of the placeholder after setting the
text placeholder.
Merge pull request #15 from ruslanskorb/master

Fixed update of the color of the placeholder.
Default realTextColor to existing color instead of overriding it with black
Merge pull request #16 from zacwitte/patch-1

Default realTextColor to existing color instead of overriding it with black
FIX: if text is nil we have to set textColor to placeholderColor otherwise placeholder text doesn´t have the right color
Merge pull request #17 from jingx23/master

FIX: if text is nil we have to set textColor to placeholderColor otherwise placeholder text doesn´t have the right color
Merge pull request #13 from NxSoftware/master

Appearance protocol support