Aws temporary credentials

This allows users to specify temporary aws credentials generated from the (STS Api)[http://docs.aws.amazon.com/STS/latest/APIReference/Welcome.html] directly or indirectly.

S3 integration is implemented per (UsingTemporarySecurityCredentials)[http://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html#UsingTemporarySecurityCredentials] guide.

All other apis are integrated via adding the `SecurityToken` header accordingly.

To use this feature, use the `ContextBuilder.credentialSupplier` method.  In your implementation, supply an instance of `org.jclouds.aws.domain.TemporaryCredentials`.

Future pull request will include an `sts` api, which will include a supplier that does this automatically.

cc @hugoduncan