Add timeout argument to transport.request

[dhermes]

See #612 (comment)

It's unclear if an httplib2.Http request can be made with a timeout if the instance wasn't created with one, though likely it could be fixed with

orig_timeout = http.timeout
try:
    http.timeout = temp_value
    do_stuff(http)
finally:
    http.timeout = orig_timeout

More problematic is when http.request has been modified (by our wrappers) and we are unsure what has been passed to transport.request (though I have an idea there).

[theacodes]

Depending on the decision in #128 we could have the guarantee that request hasn't been modified by our wrapper.

[theacodes]

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