Tables.waitForTableToBecomeActive optional params

Tables.waitForTableToBecomeActive now includes optional timeout and
interval parameters so callers do not have to wait 10 minutes
Adapt DynamoDB sample to use util.Tables

Since doesTableExist and waitForTableToBecomeActive now exist in
util.Tables, adapt DynamoDB sample to use them.
Merge pull request #167 from kenklin/master

Tables.waitForTableToBecomeActive optional params