Do not enable short key mode if debugger is not running

If debugger is not running short key mode should not be enabled
and variable that says that is is enabled should not be set.

This fixes the problem when desktop module restores shortkey
mode of long gone debuggers.
Clean things up when debugger buffer is killed

When debugger buffer is killed with C-x k the buffer is already
gone by the time we get to 'realgud-term-sentinel' and cleanup
didn't happen.

This patch adds kill-buffer-hook that cleans things up before
buffer is gone.
Merge pull request #29 from mar-kolya/kill-buffer-fixes

Kill buffer fixes - even though we have the travis failure. Will work around in next commit.