Added support for AWS PHP SDK version 2

I noticed Amazon released version 2 of their PHP SDK a while back, so I added support for it.

The changes include:
* Moving Respect/Rest to the "suggest" section in composer.json, since some people may not want to use the REST server, or wish to implement it themselves
* Added the PEAR repository definition, which is required for microsoft/windowsazure
* Bumped the Monolog version for compatibility with newer Symfony2 libraries (the new AWS SDK depends on the Symfony2 event dispatcher and Guzzle)
* Added AmazonSQSV2 and AmazonS3V2 Backend classes, which use the new library functions
* Added Base::setConnection(), which enables dependency injection (e.g. someone might want to pass an existing connection object to a backend instance)
* Added tests for the new classes and functions

What do you think? I thought of simply replacing the AWS classes, but some people might want to stay with the old SDK. Maybe do it in a different version branch?