Phase 1 of adding asset metadata saving to old Mongo

@dmitchell @cpennington Here's the first phase of my work. Look at the tests to see what is implemented and what isn't yet.
Without a __repr__, my classes output something like this:
```<xmodule.assetstore.AssetMetadata object at 0x4ae2810>```
With a __repr__, the output is:
AssetMetadata('/c4x/edX/test_unicode/asset/pic1.jpg', 'pic1.jpg', 'EKMND332DDBK', 'False', 'None', 'None', '14', '13', 'Mork', '2014-08-19 18:37:24.282000+00:00')

Good to know about __eq__, __ne__, and __hash__! I'll add them.