warn when *_PROTOCOL is a non-class

[coyotemarin]

MRJob.*_PROTOCOL is supposed to be set to a class, but our code just treats it as a callable, which makes it possible to define e.g. INPUT_PROTOCOL as a method (which is what input_protocol() is for).

This seems somewhat harmless, but I don't want people to get into this habit and have to support it later.

I also briefly considered merging INPUT_PROTOCOL and input_protocol (since as of v0.4 they have identical functionality), but it messes up the consistency with other constants (e.g. HADOOP_INPUT_FORMAT).