RAPI server: Move user file watching out, update documentation

This patch moves the code watching the users file into a
a separate class to not mix it with HTTP serving. The users
file is now driven from outside the HTTP server class.

Also the documentation is updated to mention the automatic
reloading.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>