Add an optional default_timezone to r/iso8601

The `iso8601` function fails if the string does not contain a timezone. The javascript driver's function [`r.ISO8601`](http://www.rethinkdb.com/api/javascript/#iso8601) allows an optional default timezone to be specified. Revise should do the same.

Start at the r/iso8601 entry point and compare its current behavior with the linked JavaScript driver's ISO8601 function. Trace how timezone-less strings are handled, then verify that an optional default timezone is accepted and used when the input has no timezone.

Written by the indexing model from the issue text.