Build path JRE set to Eclipse default JRE

As discussed, the original build path has a hard coded Alternate JRE system library named "jre7". This may not exist in other Eclipse environments and will cause build problems.
This pull request sets the JRE system library to the Workspace default library so it doesn't matter what the JRE is called. It does require a pre-req of JRE 7 which is common these days.