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)
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.
  ...
Added warning when calling EncryptionUtils.encrypt with data longer than keylength
Cleaned up encryption keys in the tests so there are no warning when running tests
AWS.eager_autoload! now autolaods all of AWS::Core.

Fixes #279
Removed the need to specify modules by crawling all constants when eager autoloading.
Resolved an issue with AWS::IAM::VirtualMFADevice#enable.

Passing in string values for a code like '099999' were raising
runtime errors from sprintf.  This change prevents string values
from getting passed through sprintf.  Added a test to cover this
behavior.

Fixes #288
Reverted the return value from AWS.eager_autoload!
Use HTTPS when talking to rubygems.org.
Merge pull request #289 from ab/rubygems-https

Use HTTPS when talking to rubygems.org.
Updated unit tests to not rely on return value from `eager_autoload!`.
Updated the documentation for eager_autoload! to a void return.
Replaced usage of === with == when checking for nil.
Merge pull request #280 from aws/eager-autoload-fix

AWS.eager_autoload! now autolaods all of AWS::Core.
Add specs for streaming http responses
Pending specs for more reasonable streaming behaviour

Previously, errors in subsequent chunks were caught and then the whole
request was retried, yielding (possibly) repeated chunks to the
read_block, without any indication (in the block) of this being done.

The outer client method would also not report any errors for a consumer
to check.
Disable retries as soon as any data has been yielded