Add pkey comparison function to bindings

Expose the EVP_PKEY_cmp function, so that pyOpenSSL can implement
comparison methods on PKey.
Removed argument names per review
Merge pull request #1450 from viraptor/add_pkey_cmp

Add pkey comparison function to bindings
Use explicit relative imports, don't refer to the tests package
Merge pull request #1452 from alex/use-relative-imports

Use explicit relative imports, don't refer to the tests package
Merge branch 'master' into use-full-impls

Conflicts:
	tests/hazmat/primitives/test_cmac.py