Updated README for Building with Maven and Fixed Broken Links/Commands

README now provides instructions for getting started with Git and building libSBOLj using Maven.  Previously broken links have also been fixed to point to the latest locations for the files referenced in the text.  In addition, the commands for validating SBOL files via the libSBOLj JAR file have been updated with the name currently given to the JAR file when built using Maven.  Lastly, the section on the libSBOLj API has been marked as being out of date since the specified ant commands result in an error message stating that "libSBOLj\lib does not exist."