Add StringToString transformer

In some cases we'll want to save the data from the stream directly with
no transformation needed. This will allow us to return the raw data
string from the stream

* Add new StringToStringTransformer
* Remove Record from codebase in favor of more generic interface
Sort exported fields alphabetically
Update example usage in README

* Showcase the new StringToStringTransformer as a simple way of getting
data directly from Kinesis to Redshift.