some improvements

Hi Julien,

I have made some changes to the module: 

* Deprecated __get_uniq_key by reading the MONGO_UNIQ_KEY at construction time instead of being evaluated several times.
* If MONGO_UNIQ_KEY, index is created used ensure_index.
* Added pymongo safe support to insert and upsert operations.
* Added MONGODB_ITEM_ID_FIELD to set the _id to item.

Please consider merging those changes and bumping up the module version.

Thanks,