Added support for sending "expect" and waiting for 100-continue.

Added two new configuration options to AWS.config:

* :http_continue_timeout (time in seconds, defaults to 1)
* :http_continue_threshold (size in bytes, defaults false)

If a request body exceeds the :http_continue_threshold, then the
"Expect" header will be set to "100-continue".  The http client will
then wait for a "100 Continue" response or :http_continue_timeout
seconds before sending the body.

By default the :http_continue_timeout is set to 1 seconds and the
:http_continue_threshold is disabled (set to false).

To enable 100-continues you need to do 2 things:

* Call `AWS.patch_net_http_100_continue!`
* Set `:http_continue_threshold` to a positive integer (number of bytes)

References #172