Adds ImportKeyPair

Hi - this commit adds the AWS [ImportKeyPair](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-ImportKeyPair.html) call. Right now only the [CreateKeyPair](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-CreateKeyPair.html) is implemented...

This comes from a [feature request](https://issues.apache.org/jira/browse/DTACLOUD-438) we had for the ec2 driver of the [Apache Deltacloud](http://deltacloud.apache.org) project - thanks for your consideration! It would be awesome if (assuming you agree with the commit) - after merging you could actually cut a new version of the gem so that we can consume this new functionality within Deltacloud. 

many thanks, marios