Override dict's get method

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[x] Added tests for code changes or test/build only changes
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Merges #474 and tests.

One sentence in the pydoc was a little bit misleading, so I changed it to:

Dictates whether a remote request is made to retrieve the doc, if it is not present in the local cache.

Extended the tests to cover the new behaviour based on the proposed behaviour table.

Call	Doc is local	Doc is remote	Result	Covered with tests
`db['foo']`	`True`	`True`	returns local doc	test_create_document_that_already_exists test_create_document_without_id test_create_design_document test_create_empty_document
`db['foo']`	`False`	`True`	returns remote doc	test_get_document_from_remote
`db['foo']`	`False`	`False`	returns `KeyError`	test_get_non_existing_doc_via_getitem
`db['foo']`	`True`	`False`	returns local doc	test_get_document_from_cache
`db.get('foo')`	`True`	`True`	returns local doc	test_create_document_that_already_exists test_create_document_without_id test_create_design_document
`db.get('foo')`	`False`	`True`	returns `None`	test_get_document_from_remote
`db.get('foo')`	`False`	`False`	returns `None`	test_get_non_existing_document_from_cache
`db.get('foo')`	`True`	`False`	returns local doc	test_get_document_from_cache
`db.get('foo', remote=True)`	`True`	`True`	returns local doc	test_create_document_that_already_exists test_create_document_without_id test_create_design_document test_create_empty_document
`db.get('foo', remote=True)`	`False`	`True`	fetches remote doc	test_get_document_from_remote
`db.get('foo', remote=True)`	`False`	`False`	returns `None`	test_get_non_existing_document_from_remote
`db.get('foo', remote=True)`	`True`	`False`	returns local doc	test_get_document_from_cache

New assertions to cover the new feature to:

New test cases: