rest-api: Be a bit more RESTful

 - Drop all of the container actions and instead use /state for that
   with GET/PUT.
 - Rework files to use GET/PUT.
 - Add shell examples.

Reported-by: William Grant
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Merge pull request #77 from stgraber/master

rest-api: Be a bit more RESTful