Removed dependency on BoxAPIKey.h modification in order to use the SDK.

I've removed the dependency on the BoxAPIKey.h file to specify the developer API Key.

Using the current strategy with completing a NSString constant in this file introduces problems if developers use the SDK as a git submodule or with cocoapods. In these cases you would want the SDK source to be standalone and work without modifying the source. 

The SDK key is now injected directly on the BoxRESTApiFactory by the user instead.