Add listen gem to dependencies

listen gem (https://github.com/guard/listen) requires at least version
1.9.3 of Ruby so the `.ruby-version` was bumped to accomodate.
Use the socket if it is passed in
Pull message into attribute and specify a default

If no message is found in the options then generate a default message
based on the command which will be executed.
Attempt to receive data in order to determin socket closure

Execute the `notify_command` when the socket has been closed instead of
after `dtach` is called. In order to do this attemp to receive IO on the
UNIX socket created by running `dtach -n`. No IO is actually expected
but by listening for IO it is possible to rescue an error when the
socket closes.

Some information about socket interaction was found on the Ruby-Talk
mailing list at
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/242369
Remove listen gem from Gemfile

With the discovery of listening on a UNIX socket and waiting for an
error it is no longer necessary (nor did it ever work) to listen for
filesystem events.
Bump version to 0.1.0