Add support for timezone connection option

[trevorr]

Support for the timezone connection option as specified by mysqljs/mysql:

timezone: The timezone configured on the MySQL server. This is used to type cast server date/time values to JavaScript Date object and vice versa. This can be 'local', 'Z', or an offset in the form +HH:MM or -HH:MM. (Default: 'local')

Inspired by #732 but with tests and against current master. Removes the need to run tests with process.env.TZ = 'UTC'. It doesn't set the session time_zone variable, but the user should do that if the server time zone is not the same as the requested time zone. However, since time_zone only applies to timestamp columns and date functions — and not stored datetime values — the client and server really need to agree on the time zone in advance anyway.

Fixes #15, #262, #642, #877, #888, and maybe others.

[sidorares]

Great work @trevorr !