Allow locally specifying the location of the PID file
Accept globally specifying the location of the piddir
Accept specifying the location of the PIDDIR from the environment
Rake required in dev, but left unspecified in gemspec
Remove trailing whitespace
Simply crash if PIDs directory does not already exist

It should not be the responsibility of the locking implementation to
create the directory: maybe we need specific permissions, or symlinks,
or what have you.
Make it possible to obtain the pidfile from outside

For logging purposes, for example.
Merge pull request #1 from francois/master

Enable specifying the location of the `piddir`