Refactor the interface of commands.c

Refactor the interface of commands.c

This change has two implications:

1) tree_render() will now be called precisely once for input which consists of
   multiple commands (like "focus left; focus right"). Also, the caller of
   parse_command() has to call it. This makes us able to fix tickets such as
   ticket #608 (where multiple tree_render() calls are noticable).

2) The output of a command is now a JSON array of return values of the
   individual subcommands. In the case of "focus left; focus right", this is:

   [{"success":true}, {"success":true}]

   While this is incompatible with what i3 returned before, the return value of
   commands was undocumented and therefore not subject to our API stability.