Feature/util methods

created file utils.py and test_utils.py, utils.py contains two methods build_url and validate_attr_is_acceptable which are used by the SDK. The first simply builds a url given a path and base api url which is defined in the config.py file or given directly to the method. The second, validate_attr_is_acceptable, takes a value and list of values to check against. It checks to see that the value is included in the list of acceptable values. There is an optional argument for allowing a None value. 
I think we should also test the case where allow_none is True (the default) and a None value is passed in to make sure that doesn't result in an error.

I also think that we should verify for the build_url call that if no base url is passed in, it defaults to the config setting's BASE_API_URL.