Account for new DynamoDB read pricing

Amazon announced on 2013/05/15 that a single read capacity unit will
allow you to fetch an item up to 4 KB in size; change the
getProvisionedThroughputObject method to take this into account when
calculating how many read units to provision.

Change how write unit provisioning is calculated, making it:
 - separate from the readUnit calculation
 - easy to fix if/when  Amazon changes that capacity later.

Change tests as appropriate.
Tidy up comments
Merge pull request #6 from riwsky/master

Account for new DynamoDB read pricing