Implement asAssociate() methods

[jsm1t]

Fetching data using the .asAssociate option is currently not yet supported, but required for B2B functionality. Allows fetching data based on the associate role in a business unit. (See: https://docs.commercetools.com/api/projects/associate-business-units)

[mvantellingen]

Some basic support is added via #259 but it doesn't do any permission checking yet. At minimum we should validate the business-unit.