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
Merge branch 'master' into SNSMessageHandling

* master: (159 commits)
  Fixed an issue with the stubbed responses for S3::Client#complete_multipart_upload.
  Fix undefined method 'version_id' during multipart upload
  Fix bug: default credential provider hides sub credential provider's fetching credentials error.
  missing require time lib for Time.parse is used in EC2Provider, the test passed because there is an example required the time lib
  Bump version to 1.8.1.1
  Removed REE from travis configuration, currently broken on travis-ci.org.
  Removed HTTParty from gem specification.
  Bump version to 1.8.1
  Removed gem dependency on HTTParty.
  Added support for Elastic Transcoding errors and some integration tests.
  Added a basic Elastic Transcoder client.
  Fixed documentation for AWS::CloudFront (incorrect example).
  Fixed "configuraiton" mispelling everywhere
  Spec for allowing workflow_id parameters to some decider methods
  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.
  Removed @private doc tag from AWS::IAM::Resource.
  Added #create and #create! methods to AWS::Record::AbstractBase
  Removing deprecated usage of nested Given/When/Then from features.
  Removed a few un-needed test dependencies and updated others.
  Fixed spec broken by NetHttpHandler update for network errors.
  ...
Replaced the AWS.eager_autoload! implementation.

This implementation no longer relies on the AWS.register_autoloads
hook, runs faster and is simpler.
Removed the usage of AWS.register_autoloads, now using vanilla autoload.
Removed the legacy autoloader module.
Updated AWS::Record to use autoload instead of explicit require statements to support eager autoloading.
Merge pull request #209 from aws/autoload

Autoload update
Refactor duplicate attachment code in EC2
Spelling error in load_balancer_collection.rb
Merge pull request #213 from bensie/patch-1

Spelling error in load_balancer_collection.rb
Removed trailing whitespace from the end of lines.
Removed (stale) example rails application.