Makefile rules for build/node.

The files in build/node are checked out from a previous git commit,
ie they are 'known good' versions of src/node files. Added rules to create or update
build/node when src/node/* changes. Removed the deletion of build/node upon success in
the build and added build/node to .gitignore instead.

TBR=johnjbarton

Review URL: https://github.com/google/traceur-compiler/pull/610

Closes #610.