Return all data for RDS instances

Instead of returning the endpoint address only, return the full hash as
provided by the AWS API.
Return specific normalized data for RDS instances

For Puppet to work, hash keys have to be converted from symbols to strings.
Also return database engine
Add normalize_instance_data()
Update documentation
Restore backward compatibility

hiera('rds_instances') will return the new hash data, while
hiera('rds') still returns the endpoint address only.

This allows us to update to the new functionality without breaking
existing Puppet code. The logic behind hiera('rds') can be removed
later on.
Rename #normalize_instance_data to #prepare_instance_data
Merge pull request #9 from Jimdo/rds_return_more_data

Get more properties of RDS instances