Python-arango

* Duplicated several methods in `arango.client.ArangoClient` to `arango.database.Database` to allow non-root users access to the API operations (only root user could call them before). More information on the duplicated methods and their usage can be found in the following pages:

* http://python-driver-for-arangodb.readthedocs.io/en/master/wal.html
* http://python-driver-for-arangodb.readthedocs.io/en/master/admin.html
* http://python-driver-for-arangodb.readthedocs.io/en/master/user.html
* http://python-driver-for-arangodb.readthedocs.io/en/master/async.html

* Added new method `arango.client.ArangoClient.asynchronous` to replace the soon-to-be-deprecated method `arango.client.ArangoClient.async` (due to name conflict with Python 3.7+ keyword `async`). The changes are reflected in http://python-driver-for-arangodb.readthedocs.io/en/master/async.html.

* Improved the performance for large batch commits.
* Updated method `Collection.import_bulk` with new parameters.
* Added support for ArangoDB version **3.1.21**.

* Added parameter `replication_factor` to method `Database.create_collection`.
* Added support for Python version **3.6.x** and ArangoDB version **3.1.17**.

- Added new method `Database.get_document` to allow direct retrieval of documents via their IDs.
- Updated method `Collection.all` to use ArangoDB's built-in `all` simple query instead of the export API **(this changes the method's signature and therefore is a backwards compatibility breaking change)**.
- Added new method `Collection.export`, which is equivalent to the old `Collection.all` method.

- Used the correct export cursor endpoint for `Collection.all` method.
- Fixed minor issues with unit tests.
- Fixed a few typos and improve documentation.

- Added new method `arango.client.ArangoClient.log_levels`.
- Added new method `arango.client.ArangoClient.set_log_levels`.
- Added new parameter `system` to arango.database.Database.delete_collection`.
- Improved docstrings and documentation.