Added ability to remove previously registered super properties

Added methods to remove super properties that have been previously registered, for use as an alternative to clearing all of the super properties just to change a subset of them.

This is a refresh of #15 with proper synchronization and documentation. In our use case, it is far more efficient to remove specific super properties than to clear and rebuild the entire structure. Documentation reminds the user that no longer having a value for a property will cause it to be displayed as *undefined* in Mixpanel.

The *remove* verb was chosen rather than *unregister*, which is used in the Android API. This choice was for consistency with Cocoa collection methods and also because some people would expect *deregister* rather than *unregister*. Glad to update the PR for consistency with Android if that is preferable.