Change auth#destroy to look for auth_token

Since all authenticated methods look for auth_token, it didn't make
sense for #destroy to use :id instead of :auth_token.
Move 'respond_to :json' to api/base controller

All API controllers return only JSON, so it makes sense to do this.
Use 'before_filter :verify_auth_token' for api/auth#destroy

This controller action duplicated functionality found in
verify_auth_token.
Remove timestamp field from all JSON responses

The timestamp field has little use, and no other public APIs out there
have it (except for Amazon AWS's, for which it actually makes sense).
Also, change the message JSON response to use "timestamp" instead of
"received" for the date received field.
Fix 404 error message for api/messages#show

It erroneously returned "Tag not found" instead of "Message not found"
Add API documentation for api/auth and api/devices