Arango-orm

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- Database.has_collection method.
- Examples and README updated to use ArangoClient correctly for the 4.x version.
- Fixed 10 - Collections now raise SerializationError instead of RuntimeError
when loading or dumping data to the db fails.

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- Query.first() and Query.one() methods implementation to return the first record that matches the query

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- Schema fields are now be defined inside the main model class instead of a nested _Schema child class
- Allow extra fields not present in the schema to be present in collections without any validation or type conversion
- Load and dump extra fields only if _allow_extra_fields is set to True for the collection class
- Bound db to model object. If an object has interacted with the db then it's _db attribute points to the database
- Collections now have _pre_process and _post_process methods that get called before and after data loading into the collection respectively
- Database.create_all method creates all collections, relations, graphs (with their edge definitions) that are passed onto it as a list.
- Database.update_graph creates collections, relations, edge definitions and drops or replaces edge definitions if they have changed. Does not drop any collection or relation.

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- Graph creation also supports creating indices from collection class _index attribute
- Support for passing collection create options as supported by `python-arango database.create_collection <http://python-driver-for-arangodb.readthedocs.io/en/stable/classes.html#arango.database.Database.create_collection>`_ method to database.create_colltion method

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- Support for creating indices by defining _index attribute in model definition