Added shard_key meta, so save() and update() passes shard keys to the pymongo query. Also made shard key fields immutable. Closes #388 and #389
@@ -89,3 +89,6 @@ that much better:
* Ankhbayar
* Jan Schrewe
* David Koblas
* Crittercism
* Alvin Liang
* andrewmlevy
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