Start work on mongoengine port.
More work on mongoengine port

 - admin_app now reads trgtb-ENGINE for data (which is where we've got a copy of the DB for porting work)
 - added engine classes for CorpusItem and Link
 - added some functionality to Feed, including the beginning of a data migrator
 - admin_app tweaks to not blow up on the migrated data
mongoengine porting part 3

- Drop trying to use the same collections
- Add conversion code to load the new collections with the old data
- Convert the admin app to read the new collections
- Add feed-following to the mongoengine code
mongoengine: cleanup admin CLI tool

- remove the corpus commands, since they are to be superceded by the admin_app
- cleanup feed commands
- convert feed commands to feed_engine
mongoengine: Fix typo in randblog.rss.admin
mongoengine: Move back over to main DB

I'm now looking at the new collections and comparing to make sure the code
is still operating properly feed-following wise.
Specify newer versions in requirements.txt (now minimum, rather than exact)
mongoengine: fix title change entry duplication bug
mongoengine: move randblog.rss pieces to new version, drop conversion code
mongoengine: convert randblog.crawler piece to new
mongoengine: convert randblog.corpus to new code
mongoengine: remove last bit of conversion code
mongoengine: Add appropriate __all__ fields to various modules
mongoengine: Eliminate code for old DB access, Clean up old commands
Merge pull request #2 from mghlarsen/engine

Basic MongoEngine port

This fixes #1, although with some regressions in stats and text generation. I've opened issues to track fixing these problems.