Add the dependencies feature

- update the tests
Merge branch 'dependencies' of github.com:Lujeni/cerberus into Lujeni-dependencies

Conflicts:
	cerberus/cerberus.py
	cerberus/errors.py
Enclose field name with quotes.
PEP8
Docs for 'dependencies' rule
Added 'dependencies' rule (Lujeni)
Merge branch 'Lujeni-dependencies'