Add a validator for schemas of dictionary keys

Allows defining a schema for all the keys of a dictionary with the
keyschema schema key. The dictionary may have arbitrary keys, but
all of these must validate against the given schema.
Document keyschema schema key
Merge branch 'key-schemas' of github.com:kynan/cerberus into kynan-key-schemas

Conflicts:
	docs/index.rst
Improved 'keyschema' documentation and examples.
'keyschema' validation rule.
Florian Rathgeber
Merge branch 'kynan-key-schemas'