Lots of changes

* Partially implements new Snapchat::register method.
* Fixes endpoint and parameter order in Snapchat::findFriends.
* Partially reverts changes to Snapchat::setStory from
20b36dcedba140344f9fade4ab2f6324655c01ee.
* Fixes Snapchat::getStory and Snapchat::getStoryThumb not using the new
request format from 75b19ad02e5d28cc52984b0b31d997ffa085d082.
* Adds the new method Snapchat::deleteStory.
* Adds support for optionally returning the response headers in
SnapchatAgent::post, SnapchatAgent::get and SnapchatAgent::request.
* SnapchatAgent::request now uses the same request headers as the target
application version.