Added is_classmethod=True (use caefully)

Good for proxy objects (I use it for RPC with twister.net.co)
where object's "state" is static connection details
Added a SkipCache(message,retval) exception

README includes a "real life" code sample
renamed to ignore_instance, added tests

`is_classmethod` was indeed a confusing term
Merge branch 'classmethod' into skip-cache

Conflicts:
	functioncache/__init__.py
added test_skipcache
added .gitignore
Merge pull request #3 from thedod/skip-cache

Skip cache