raise mongodb resource limits

MongoDB 2.3+ warns if file descriptor limits are too low. In newer versions of Mac OS X both soft and hard file descriptor limits are are 256 by default so this new proposed plist raises the limit to 1024 in order to make MongoDB happy.

I've tested installing the new formula and loading the new plist locally. It would be great if we could get this in before 2.4.0 is released. It is being proposed now as 2.4.0-rc0 is currently available.