Test selector arguments using a dedicated method, not -performSelector:.

-performSelector: is part of the NSObject protocol, to which NSProxy's
conform. This means that when -performSelector:@selector(lowercaseString)
is invoked on a mock or recorder, the invocation handled isn't
for -performSelector", it is actually for -lowercaseString--they will try to
actually perform the selector, and only record an invocation when they fail.
Add an "any selector" option to OCMArg.

Although SELs are typedef'd pointers, the -anyPointer option could not
be used because NSInvocation+OCMAdditions would try to get
the selector object-value as a string--which would be nil, for a pointer which
wasn't a selector. I end up changing NSInvocation+OCMAdditions
to box SELs as pointers anyway, but I figured a more explicit interface
to OCMArg than -anyPointer couldn't hurt.
Merge pull request #7 from inkling/selector_arguments

Improvements to handling of selector arguments.