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.
Optimize import statements
Use Try rather than Injection for long conversion

This method is a bit more resistant to errors and was suggested
by @johnynek
Use a FuturePool to execute API requests.

The default Dynamo client uses blocking requests which means
we need to execute the api calls on background threads so as
to not block the main thread.

At this point it uses a FuturePool backed by a
 hard coded fixed size thread pool. Work can be done in the
future to make the size of the threadpool configurable
Add copyright headers to dynamodb package
Make Future thread pool size configurable
Fix typo in credential ordering
Small refactor for more idomatic Scala
Merge pull request #144 from sharethrough/add_storehaus_dynamodb

First version of DynamoDB Store