Dual api versions

Refer to Issue 65:

  https://github.com/mboudreau/Alternator/issues/65

Amazon created a new specification for the DynamoDB API that is NOT backward compatible with the earlier version. The earlier version of the API is still supported but is deprecated.

Note however that only the features that were already available thru the original API version are supported.

When processing against the newer API protocol, Alternator simply maps the request objects to the original format and calls the pre-existing logic. It then maps the result to the new API protocol format. Any exceptions are also remapped from the *dynamodb* namespace to the *dynamodbv2* namespace.

This revision also implements the BETWEEN, BEGINS_WITH, and CONTAINS scan filters and corrects the comparison direction for the GE scan filter.