Implement locking

[jaxxstorm]

The README suggests using flock for locking, however there's an issue with using this method:

1. If you want a exclusive lock, flock will wait for the lock to be available and then run
2. If you want to avoid this behaviour, you'll have to use the -n non blocking mechanism. Unfortunately, this will always return exit code 1. This isn't optimal.

With this in mind, I think it's best to build in locking to sensu-wrapper