Add AWS::SNS::Message and build-in authenticity-checking.

AWS::SNS::Message#authentic? will return true if the message is cryptographically authenticated
(see http://docs.amazonwebservices.com/sns/latest/gsg/SendMessageToHttp.verify.signature.html).
To-do:
* support subscription-confirmation and unsubscribe messages; this just supports
notifications.  Should just mean changes to canonical_string method.
* decide whether it's really OK to return false in the event of the check throwing an error (for
example, if the cert-download fails too often or times out).  Might be best to leave that up to
the caller?
Add support for Message finding out its own origin.

Introduce the concept of a message's origin, and introduce a partial implementation of
an AutoScaling originator (partial, in that there's not coverage for all of the data that
auto-scaling will include in the SNS' body).

The origin-selection currently works simply; there's only one originator at present, so
it's checked in-line in Message's initializer, rather than that iterating through the
available originator modules.
Merge branch 'master' into SNSMessageHandling
revert to ruby 1.8.x hash-syntax

(CI runs tests using ree-1.8.7)
add support for excluded resources
fix NotResource case
fix alias
fix json generation
fix typo
Allow signal_external_workflow_execution and request_cancel_external_workflow_execution to accept a String workflow id as the first parameter. This is consistent with the current documentation.
Spec for allowing workflow_id parameters to some decider methods
Fixed "configuraiton" mispelling everywhere
Added a basic Elastic Transcoder client.
Added support for Elastic Transcoding errors and some integration tests.
Merge pull request #139 from rmm5t/fix-doc-typos

Fix "configuraiton" mispelling everywhere
Merge pull request #136 from sblakey/master

Allow String workflow_id as first parameter for certain decider methods.
Removed gem dependency on HTTParty.

The AWS::Core::Http::HTTPartyHandler has long since been replaced
as the default handler.  This commit removes the gem dependency
on HTTParty and removes the handler.
Merge pull request #142 from aws/no-httparty

Removed gem dependency on HTTParty.
Merge branch 'private-elastic-transcoder' into release/1.8.1
Bump version to 1.8.1