Upgrade to newer version of async-http library

 - Makes RestHandler and ContainerRestHandler abstract to force subclasses
   to implement dataReceived and failure methods
 - Adds an abstract JSON handler to marshal the several different
   onFailure methods into a single (abstract) failure method
 - Switches the HMAC signature code to throw explicit checked exceptions,
   instead of just throwing Exception, so that we don't accidentally
   silently swallow runtime exceptions
 - No longer calls the handler's onFailure method when creating the HMAC
   signature fails.  This is very unlikely to happen in production, and if
   it does happen it will likely happen on every request, so there is no
   nice way to handle it in the UI layer anyways.
Add sane timeout to HTTP requests

The default timeout is 10 seconds, which seems a tad long
Merge pull request #175 from maurizi/upgrade-async-http

Upgrade async http