URL builder for the FDSN webservices.
Built as a separate function to enhance testability.
>>> build_url("http://service.iris.edu", "dataselect", 1, "application.wadl")
'http://service.iris.edu/fdsnws/dataselect/1/application.wadl'
>>> build_url("http://service.iris.edu", "dataselect", 1, "query", {"cha": "EHE"})
'http://service.iris.edu/fdsnws/dataselect/1/query?cha=EHE'