Finish implementing the HttpActionHandler

[mmarum-sugarcrm]

Finish implementing the HttpActionHandler. This includes optional features like expects, returns, requires, prefers, and target. A big part of this task is implementing the HTML form POST feature where an action handler defines the input to collect from a user which needs to be collected from user via form inputs prior to executing the action.

See section 3 of spec
http://www.ietf.org/id/draft-snell-activitystreams-actions-03.txt

Dependent on #5

[mmarum-sugarcrm]

This task likely needs to be decomposed into sub-tasks.