add support for amazon temporary credentials

added api `sts` and `aws-sts` referenced [here](http://docs.aws.amazon.com/STS/latest/UsingSTS/UsingTokens.html)

you can make temporary credentials by doing the following:

```
RestContext<STSApi, STSAsyncApi> stsContext = ContextBuilder.newBuilder("aws-sts")
          .credentials(accessKey, secretKey)
          .build();

TemporaryCredentials creds = stsContext.getApi().createTemporaryCredentials(durationSeconds(MINUTES.toSeconds(15)));

s3Context =  ContextBuilder.newBuilder("aws-s3")
          .credentialsSupplier(Supplier.<Credentials> of(creds))
          .build();

```