AWSSessionCredentials support

AWS allows to use session credentials with keys and session token.
But AWScala ignores AWSSessionCredentials,  so i got this error.

```
Exception in thread "main" com.amazonaws.services.s3.model.AmazonS3Exception: The AWS Access Key Id you provided does not exist in our records. (Service: Amazon S3; Status Code: 403; Error Code: InvalidAccessKeyId; Request ID: XXXXX), S3 Extended Request ID: XXXXXX
        at com.amazonaws.http.AmazonHttpClient.handleErrorResponse(AmazonHttpClient.java:937)
        at com.amazonaws.http.AmazonHttpClient.executeHelper(AmazonHttpClient.java:506)
        at com.amazonaws.http.AmazonHttpClient.execute(AmazonHttpClient.java:273)
        at com.amazonaws.services.s3.AmazonS3Client.invoke(AmazonS3Client.java:3660)
        at com.amazonaws.services.s3.AmazonS3Client.invoke(AmazonS3Client.java:3612)
        at com.amazonaws.services.s3.AmazonS3Client.listBuckets(AmazonS3Client.java:672)
        at com.amazonaws.services.s3.AmazonS3Client.listBuckets(AmazonS3Client.java:680)
        at awscala.s3.S3$class.buckets(S3.scala:38)
        at awscala.s3.S3Client.buckets(S3.scala:239)
        at com.github.tanacasino.app.App$.main(App.scala:16)
        at com.github.tanacasino.app.App.main(App.scala)

```