Re-enable RedshiftSourceSuite using a new InMemoryS3AFileSystem

NOTE: check out https://github.com/spark-redshift-community/spark-redshift/pull/22 for previous reviews. I created a new branch to polish the commit history.

Fixes #20 Notes to ease reviews:

RedshiftSourceSuite is taken from databricks/spark-redshift, this Suite had been temporarily disabled, I finally got to put it back.
InMemoryS3AFileSystem is inspired to the old deprecated InMemoryS3NFileSystem and the future hadoop 3.1.0 MockS3AFileSystem. I also inspired to the actual S3AFileSystem.
SeekableByteArrayInputStream is taken from apache/accumulo at https://github.com/apache/accumulo/blob/master/core/src/test/java/org/apache/accumulo/core/file/rfile/RFileTest.java under the Apache 2.0 License.
I also added tests for my fake InMemoryS3AFileSystem

@davidmarin for review

spark-redshift-community / spark-redshift