Add client configuration parameters to the credentials map

This change allows you to configure the Amazon clients with the [ConfigurationBean](http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/ClientConfiguration.html). I'm going to use it to configure a proxy but it's useful if you need finer control over the configuration of the clients.

I've added some words to the readme to describe the additional configuration and tests to check that the client works as expected. It's difficult to write tests that require a proxy, but I've installed the snapshot version and verified that it behaves as expected.

I've not run all of the tests because I need to set up my environment to look like yours. I've run the tests and they run in the same was as the master branch.

I've also just noticed that my editor has removed whitespace at the end of some of the lines. If that's a problem I'll submit another pull request with exactly the modifications required for this change.