Mongokit

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* add autoref support to belong_to (delete cascade)
* changing collection dynamically
* add immutable field (python tuple support)
* add direction and ttl to index support
* add connection sharing support
* add json import/export for MongoDocument
* full relation support (related_to)
* add long type support

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* add autoref support (thanks to bwmcadams)
* add mongodb index support (thanks to marcammann)
* adding CustomType (original idea from Phillip Oldham)
* support now all type of subclassed supported type
* add "delete cascade" feature
* add the possibility to skip the validation layer for more performances
* fix issue while passing queries to fetch() and update tutorial
* self._collection must not be None in __init__
* fix 11 - pylons_env extension documentation typo
* add more complete test + docstring
* fix issue 9 - bug with custom_types and nested dict in list