Added no-cache headers to all responses

(This relates to https://github.com/deployd/deployd/issues/202)

User-collection now sets the following headers to all responses: 

Cache-Control: no-cache, no-store, must-revalidate
Pragma: no-cache,
Expires: 0

According to http://stackoverflow.com/questions/49547/ these are the correct headers to set to disable caching in most browsers.