Basic setting support for Companies

Based on the discussion in #10 

The basic usage to set a user's companies is...

	user = User(email="foo@bar.com")
	user.companies = [
    		{"id":6, "name": "Intercom", "created_at": datetime.now()},
    		{"id":7, "name": "GitHub", "created_at": datetime.now()}]

	# OR (Inspired by the ruby client)
	user.company = {"id":6, "name": "Intercom", "created_at": datetime.now()}

As the Intercom API does not support returning company info, an AttributeError is raised if the getter of the company/companies properties is called.
Added in test coverage for the new Company object and company setters.

Let me know if the test coverage is OK, or if I have missed anything and I'll glad work on improving/adding it!