Let user decide how to register the elarm_summary_sup for the alarm server

This allows not to require gproc to be started before starting elarm.

* Add test suite for different ways of registration of the
  elarm_summary_sup process;
* Also use the Opts argument in elarm:start_server/1.

TODO Avoid asking the user to pass the name of the elarm_summary_sup
     when calling elarm:subscribe_summary/3.