Add high level methods to AWS::DynamoDB::Table

Adds item_count and size_bytes to the Amazon DynamoDB high level
abstraction layer. Relevant Cucumber and RSpec tests included to
make the process of adding these for later releases a little easier.
Make AWS::DynamoDB::Table tests support 1.8.7

Ruby 1.8.7 doesn't support Time.new with parameters. Using Time.now
instead since that is sufficient for the cases covered.
Merge pull request #341 from aws/dynamodb-describe

Add high level methods to AWS::DynamoDB::Table