Ignore worksheet directory 

Since Scala IDE V3.0 has worksheets which are like a REPL session on steroids. For the worksheet to function `*.scala` files get created in a hidden `.worksheet` directory. As these are auto-generated the directory should be ignored.
Merge pull request #864 from oschrenk/patch-4

Ignore worksheet directory
Merge branch 'master' into add-common-linux-compile-file-extensions