Prevent clearing label background when it's set.

I.e. prevent clearing `label.backgroundColor` when it's neither unset
nor white.
Add UIButton *-selected-highlighted states and missing text-shadow-color-disabled
Added a NUI-wide constants file and replaced the first ones
Replaced explicit class comparisons with 'isMemberOfClass'
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 pull request #159 from mirego/add-button-selected-highlighted

Add UIButton *-selected-highlighted states
Merge pull request #164 from Hecktorzr/constants_replacement

Added a NUI-wide constants file and replaced the first ones
Merge pull request #165 from Hecktorzr/class_comparison

Replaced explicit class comparisons with 'isMemberOfClass'
Replaced 'applied flag' with simpler accessor
Included classes not inheriting from UIView in the BOOL flag refactor
Merge pull request #166 from Hecktorzr/nui_properties

Replaced 'applied flag' with simpler accessor
Version 0.4.0
Merge remote-tracking branch 'upstream/master'
correct a property name