Initial Open-Source Release of CueSheet

This marks the first open-source release of CueSheet. As summarized in README.md, CueSheet is a framework for writing Spark 2.x applications, with the following notable characteristics:

• No need to use shell scripts at all
• Minimalistic API for building Spark applications
• Separating configuration and business logic nicely
• Launching/Debugging directly from IDE during development
• Small deployment payload generated in runtime, which can then be installed in HDFS for production use
• Providing one-liner for integrating with job schedulers