Support holding a shared lock on a designated file forever

This is to support running resque-pool inside of upstart while allowing
for zero-downtime restarts.  It enables the use of this strategy:

http://orchestrate.io/blog/2014/03/26/tip-seamless-restarts-with-unicorn-and-upstart/

The idea is that when the pool starts, it opens a shared lock that it
will hold forever.

In the envisioned use-case, upstart is responsible for restarting a
resque-pool if it fails.  When resque-pool daemonizes, upstart keeps
track of the process id.  However, to ensure no downtime, we do not stop
and start the pool.  Instead, when a deploy happens, a new resque-pool
instance will start and kill the old one after it is ready to fork
workers.  This would ordinarily cause upstart to detect that the
original process has died and relaunch it, which is not desired
behavior.  If the upstart init script tries to obtain an exclusive lock,
this attempt will block while any pool instance is still running,
including ones other than the initial process.
Merge pull request #2 from ShippingEasy/shared_lock

Support holding a shared lock on a designated file forever