Add --version option

There's no way to get the version without starting the db. As pretty much every command line tool has a way to get the version without running it, it would be nice to have this for mongod.

Something like:

$ ./mongod --version
db version v1.0.0, pdfile version 4.4
git version: afe21e02c11f9a923ab1c95edf6fdd95b9a4a51e