use uritemplates

Use uritemplates instead of cleanPathString to correctly encode the
URL path.

We follow the Google Go API here. See
https://code.google.com/p/google-api-go-client/ and especially
the subdirectory
https://code.google.com/p/google-api-go-client/source/browse/#hg%2Fgoogleapi%2Finternal%2Furitemplates