Major performance und memory usage improvents

This version is much more stable on bigger collections and used a lot less memory. Instead of reading an entire collection it once, this release queries the mongodb in chunks. The chunksize can be configured.
Furthermore, the logging has been improved and gives much more details about what's going on in the background.

Although this release has improved a lot, don't expect any magic. Reading the items from mongo can be slow, depending on the size of the collections. The insert queries however are pretty fast.