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
Slightly more kosher selection handling fixes #18
Merge pull request #76 from imagine/master

Slightly more kosher selection handling fixes #18
Added call to super setBackground in SWTableViewCell
Merge pull request #78 from dcwither/master

Added call to super setBackground in SWTableViewCell
Fixed subviews inside cell not clickable bug
Merge pull request #80 from ethan-fang/cell-weak-reference

Fixed subviews inside cell not clickable bug