sped up mongo.cursor.to.data.frame

This function was using a very inefficient copy-and-append pattern which will
quickly use all available memory.

Please see http://www.burns-stat.com/pages/Tutor/R_inferno.pdf (Circle 2,
page 12ff) for more about why this is a bad idea and how to fix it.
Merge pull request #15 from dtenenba/master

speed up mongo.cursor.to.data.frame - old solution was a temporary hack. We still have the problem that this process will not work for most NoSQL data structures.