Added allowDiskUse option in mongodb node

[thobhanifreddy]

I have added allowDiskUse option by solving part of issue #308.

It was also discussed at stackoverflow

[dceejay]

Thanks for this. Why be so specific ? Are there not other possible options to pass in ? eg batchsize ? Why not just pass in a msg.options property and (in your case) set it to {allowDiskUse:true} ?
It would also make the code simpler '-)

[hardillb]

Also please do not check in .DS_Store files