Support the continueAfterTimeout parameter

[healarconr]

Implements support for the continueAfterTimeout parameter mentioned in #29 by adding a new optional argument to the aurora_data_api.connect() function called continue_after_timeout. This argument is passed to the AuroraDataAPIClient and AuroraDataAPICursor and is used inside the cursor to add an additional execute_arg.

The corresponding unit test is skipped if an environment variable TEST_CONTINUE_AFTER_TIMEOUT is not True (it is considered False if it does not exist) to make it an opt-in test. The unit test is also skipped if using_mysql because the implementation uses the pg_sleep() function that does not exist in MySQL. An implementation of this test for MySQL could be added. I could not add it because I do not have an Aurora MySQL DB that I can access easily.

[kislyuk]

Thanks!