Added +canMatchSubject: to all "haveReceived..." matchers to filter non-KWSpy subjects.

Based on comments in the code, I may have incorrectly previously
written this as an instance method, and found it to do nothing, so
I removed. None of the included matchers use it, but while looking
at the verifier code, I found that the method was in fact used, so
I've added it back. I'm not 100% sure that all paths to using a
matcher normally will go through this test, so I've left the type
check in the "evaluate" method as well, but it's possible that it
might be unnecessary.
Tweaks to description messages for "haveReceived..." matchers.