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

Added two new configuration options to AWS.config:

* :http_continue_timeout (time in seconds)
* :http_continue_threshold (size in bytes)

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 0 seconds (will not
delay) and the :http_continue_threshold is 1MB.

References #172
Fixing a bug in Net::HTTP via monkeypatch to better support 100-continues.

References #172
Disabling Expect 100-continue by default.

Unable to support Ruby 1.8, so disabling this new behavior by
default to ensure a consistent experience.