[UI] Configure the compose controller subclass for any given sharer

Currently, there is no way to change the compose view without changing the sharer class directly. 
This change will allow people to configure the compose controller subclass to be used for different type of sharer by overriding [DefaultSHKConfigurator  SHKComposeControllerForSharerClass:] in their own configuration subclass. 

Example:
```objective-c
- (Class)SHKComposeControllerForSharerClass:(Class)sharerClass {
  if ([sharerClass isSubclassOfClass:[SHKFoursquareV2 class]])
    return NSClassFromString(@"SHKFoursquareV2CheckInForm");
  else
    return NSClassFromString(@"SHKFormControllerLargeTextField");
}
```