Fix timestamped box name.
Generate a semver string for box builds.

Thanks to the omnibus-ruby project for the inspiration:
https://github.com/opscode/omnibus-ruby/blob/master/lib/omnibus/build_version.rb
Load an external local.sh via Vagrant if it exists.

This allows a user of the box to place a local.sh file in the same
directory as the project's Vagrantfile which will be executed during the
local.sh hook section in stack.sh. Note that the local.sh file will need
to be set as executable.
Update README with box details.