Possible bug in isAvailableForServiceType

It seems like SLComposeViewController isAvailableForServiceType returns
true when the given type is not one of the SLServiceType* constants defined
in SLRequest. To circumvent this we first check if the type is in the
defined constants and return false if not
Merge pull request #262 from nhoffmann/master

Possible bug in isAvailableForServiceType