Complete the test suite by covering all Steps on Guzzle 4.

- Upgraded send_request feature for 6 HTTP/1.1 verbs.
- Upgraded set_header feature.
- New story: test POST/PUT/PATCH content.
- New story: test POST form data
- New story: test POST raw data.
- Require PHP 5.4 and Guzzle 4.
- With the test app enabled, all tests pass from testapp/.
Made tests pass on Travis. Remove extra debugging info.

- Run the PHP built-in server on Travis, since we require 5.4/5.5.
- Update behat.yml accordingly
- Fix test app input handling
- Ignore PHP 5.6, which reformats REQUEST strangely
WIP
Test the extension directly, not the test app.
Use Request object, not $_REQUEST. Test on PHP 5.6.
Update for Behat 3.0, plus nitpicks.
Removed the logger sending output to error_log

This makes the output of the build totally unreadable.
Cleaned the extension

The CLIENT_ID constant in the extension now really contains the id of the
guzzlehttp client.
Cleaned the coding standards
Renamed the extension class to support the shortcuts
Renamed the WebApiAwareContext to ApiClientAwareContext