patch inlineCallbacks to preserve functions' argpsecs

(cherry-picked from nine)
Add Steps to perform HTTP requests
Formating of the doc
Modification after review comments
port HTTPStep to txrequests

This makes this step asynchronous and suitable for buildbot

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
http step cleanup and test

- add GPL header
- change coding style
- add tests

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Merge branch 'txrequests' of git://github.com/tardyp/buildbot