Open files in a single instance by default

Having all files open in a single instance is helpful in some
situations (see issue #17).  Implement single instance support (enabled
by default) which can be controlled via the command line: --servername
specifies the name of the instance; --noremote can be used to disable
the mode.

* Add class DisplazServer to listen on a local socket, and turn incoming
  connections into command strings.

* Rudimentary newline-separated message format supports a single
  command, OPEN_FILES file1 file2 ...