Added in basic support for Companies based on the dicussion 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()}]

	# 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 rasied if the getter of the company/companies properties is called.
Added in test coverage for the new Company object and company setters.