Add a silent mode to epdb server

[wfscheper]

The epdb.serve() and epdb.serve_post_mortem() functions now take a 'silent' keyword argument. When set to True, the server does not print any information. When using an Epdb object directly, the same behavior can be enabled by setting the Epdb.silent_server attribute to True before calling Epdb.serve() or Epdb.serve_post_mortem().

Resolves #9