Multiple API versions for AWS::DynamoDB::Client

Previously you had to choose what API version to use with Amazon DynamoDB by constructing one of two client classes, `AWS::DynamoDB::Client` or `AWS::DyanmoDB::ClientV2`.  This was confusing and does not scale well with possible future API versions.

This pull request makes it possible to access multiple API versions from AWS::DynamoDB::Client.

```ruby
AWS::DynamoDB::Client.new
#=> AWS::DynamoDB::Client::V20111205

AWS::DynamoDB::Client.new(:api_version => '2012-08-10')
#=> AWS::DynamoDB::Client::V20120810
```

Please note that it defaults to the oldest API version for backwards compatibility.  You can change the global default API version using `AWS.config`.

```ruby
AWS.config(:dynamo_db => { :api_version => '2012-08-10' })
AWS::DynamoDB::Client.new
#=> AWS::DynamoDB::Client::V20120810
```