Option to remove leading @ signs from top-level fields

Mongo 2.4 includes the V8 Javascript engine, which does not permit leading "@" in identifiers. This causes a problem for running Mongo DB map-reduce against collections containing documents inserted by logstash.

 "atchop" option makes mongodb output remove leading "@" before adding events to MongoDB as documents. Defaults to leaving leading "@" alone.

This change was written a couple of months ago. I ended up moving away from Mongo DB map-reduce, so haven't needed it, but it was requested on the logstash-users mailing list by paolo.peruggini@gmail.com