Support the "Expect" header and "100-continue" responses

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_threshold` defaults to `false` (which disables this feature).  The `:http_continue_timeout` defaults to 1 second, but only takes effect when the request body exceedes the `:http_continue_threshold`.

Due to a bug in Net::HTTP, this feature will only work with a patch.  You can monkey patch Net::HTTP with the following:

```
AWS.patch_net_http_100_continue!
```

Please note, this feature does not work in Ruby < 1.9.3