Merge branch 'features/optional-deps' into develop

This includes:
- Much better variant support (+debug/-debug)
- Optional dependency support (depends_on(... , when='<condition>')
- New config file format (YAML in ~/.spack)
- New Spec format (YAML in $prefix/.spack/spec.yaml)

bin/spack

@@ -107,6 +107,10 @@ def main():
     tty.set_debug(args.debug)
     spack.debug = args.debug
 
+    if spack.debug:
+        import spack.util.debug as debug
+        debug.register_interrupt_handler()
+
     spack.spack_working_dir = working_dir
     if args.mock:
         from spack.packages import PackageDB