Optional timeout

[moppymopperson]

This is proposed solution to #67. The newly added timeout parameter is None by default, so the default behavior remains unchanged, but gives the option to add a timeout to those who wish to use one.

Originally I had added a timeout variable to the API class, but after thinking about it for awhile, I believe making the timeout an argument on public_query and private_query methods is a better solution, and this PR adopts that stance. Having to specify the timeout on each request is a little bit more verbose, but makes it easier to specify timeouts independently and dynamically for each endpoint.

[veox]

LGTM!

IMO having a timeout variable on the API class, to specify a default timeout, in addition to the "arguments" approach, is the way this will end up; especially taking into account the "retries" implementations in PR #65. This might need some thought, though, and introduction of kwargs...

Maybe next year. :)