Use npm scripts instead of grunt

Fixes #5

Instead of starting the sites with grunt, vagrant-solidus now runs the Solidus bin directly. Each site can also optionally expose 2 npm scripts in `package.json`: `compile-assets` and `watch-assets`. In short, when starting a site, vagrant-solidus will:

1. Run the `compile-assets` script
2. Run the `watch-assets` script and start the Solidus server in parallel

The following Solidus config variables are available inside the scripts:

* `process.env.npm_config_port`
* `process.env.npm_config_livereloadport`
* `process.env.npm_config_logserverport`

Note: solidus-site-template still uses grunt, so grunt-init is still used by vagrant-solidus. See the related changes: https://github.com/solidusjs/solidus-site-template/pull/20

Note: this is a breaking change