Add sigtermTime parameter to RemoteShellCommand

This pull request adds a `sigtermTime` parameter to `RemoteShellCommand`. Given a number of seconds, the buildslave will try to kill the process with SIGTERM and wait `sigtermTime` seconds before killing the process with SIGKILL. If `sigtermTime` is `None`, the process will immediately be killed with SIGKILL. The default value for `sigtermTime` is `None`.

This is in response to [trac ticket #751](http://trac.buildbot.net/ticket/751) and is a follow up to #824.