Add support for milliseconds timeout

Hey!

According to the PHPDoc, the timeout can accept a float so, this PR adds support for milliseconds timeout. Basically, `CURLOPT_TIMEOUT_MS` has been introduced in curl 7.16.2 so, if it is defined, it is better to rely on it as it is more precise otherwise we rely on `CURLOPT_TIMEOUT` as it is currently.
@egeloen Does it make sense to floor this value so we don't end up with a gnarly partial MS?