Performance optimizations

I’ve added an instance method `- (void)setAppearanceWithBlock:force:`.
The block passed as an argument is executed only once. Thanks to that
`tableView:cellForRowAtIndexPath:` is executed ~5 times faster.
Checkout updated Readme.
Merge pull request #75 from wczekalski/master

Performance optimizations
Updating podspec
Merge tag '0.2.3'

Version 0.2.3