Added singleton support so that no need to create property in application delegate.

[Janak-Nirmal]

[mikhchi]

just cloned repo and learning the design of theme loader but adding singleton pattern makes sense.

[scompt]

I don't have an opinion about whether a singleton is necessary or not, but if you do implement the singleton, it might be better to do it using dispatch_once, which seems to be Apple's preferred way.