Add --version command line option.
Introduce a VERSION variable for use with `--version`.

We need the VERSION variable because we can't guarantee the script will
be run from inside the shpec repo and therefore the VERSION file may not
exist.

`Makefile::release` will update the VERSION variable in bin/shpec.
Merge pull request #29 from twe4ked/version-command-line-option

Add --version command line option