stubbed FongoDB.getStat implementation

[connollyst]

Fixes issue #352 where calling FongoDB.getStats() would throw

com.github.fakemongo.FongoException: Not implemented for command : { "dbStats" : 1 , "scale" : 1}

Now it returns a CommandResult with an empty body. This could be expanded to return some real stats about Fongo, but at least now it doesn't error out.