Fix #104, #54: issues with overlong shebang in deep directories.

This does several things:

- Add `sbang`: a script to run scripts with long shebang lines.
  - Documentation for `sbang` is in `bin/sbang`.

- Add an `sbang` hook that filters the `bin` directory after install
  and modifies any scripts wtih shebangs that are too long to use
  `sbang` instead.

- `sbang` is at the top level, so it should be runnable (not much we
  can do if spack itself is too deep for shebang)
  - `sbang`, when used as the interpreter, runs the *second* shebang
    line it finds in a script.
  - shoud fix issues with too long shebang paths.