First version of DynamoDB Store

I modeled storehaus-dynamodb off storehaus-memcache which uses
a base client  Store[String, ChannelBuffer].

There is a base store that extends Store[String, AttributeValue]
where AttributeValue is the AWS SDK specific data type that you
use for requests to Dynamo.

DynamoStringStore and DynamoLongStore use ConversionStore
to provide a more user friendly, native type interface to
DynamoDB. DynamoLongStore is also mergeable and can be used
for incrementing atomic counters in DynamoDB.

Still needs documentation which I will add, but I wanted to get expose my work
for some feedback. 

I could also pull the Bijections/Injections out of this codebase and into the Bijections codebase but I didn't 
want to block this pull request waiting on a version bump in Bijection. I figured I could submit a separate 
pull request to Bijection and then remove them from this code base once the new version of Bijection has 
been published.