Prevent clearing label background when it's set.

I.e. prevent clearing `label.backgroundColor` when it's neither unset
nor white.
Merge pull request #149 from benjie/patch-1

Prevent clearing label background when it's set.
Fixing an issue that causes a crash when an observer messages an object which has been deallocated.
Merge pull request #169 from mutualmobile/master

Fixing an issue that causes a crash when an observer messages an object ...
Version 0.3.5
Merge remote-tracking branch 'upstream/master'