AWS::DynamoDB::Client now supports both Amazon DynamoDB API versions.

`AWS::DynamoDB::ClientV2` is now deprecated.  Instead of using this class, you can
construct a client of a specific version like so:

    ddb = AWS::DynamoDB::Client.new(:api_version => '2012-08-10')

Constructing a client without an API version returns a client of the oldest
version.  This ensures existing users of AWS::DynamoDB::Client will not experience
runtime errors from the differences.

This works by turning AWS::DynamoDB::Client.new into a factory method.  It returns
an instance of Client::V20111005 or Client::V20120810 based on the :api_version
option.
Added documentation to the versioned DynamoDB::Clients.
Documented the DynamoDB client with instructions on using the :api_version option.
Fixed an issue constructing clients from their versioned class.
You can now configure a global default API version.

If you want to specify the default API version, you can do so
using AWS.config:

    # normally defaults to 2011-12-05
    AWS::DynamoDB::Client.new
    #=> AWS::DyanmoDB::Client::V20111205

    AWS.config(:dynamo_db => { :api_version => '2012-08-10')

    # now defaults to 2012-08-10
    AWS::DynamoDB::Client.new
    #=> AWS::DyanmoDB::Client::V20120810
Udpated documentation reference to the DynamoDB constructor.
Merge pull request #298 from aws/ddb-api-versions

Multiple API versions for AWS::DynamoDB::Client