Resolve absolute plugin paths for local filesystem

When `grunt-phonegap` is installing plugins from the local filesystem, the working directory is `phonegap.config.path`, not the base project directory [containing the Gruntfile (`process.cwd`)](http://gruntjs.com/creating-plugins#avoid-changing-the-current-working-directory:-process.cwd).

I believe this is confusing to most users. This update resolves the absolute path from `process.cwd()` for plugins instead.

Such a change will break compatibility for existing users with local plugins specified as relative paths from `phonegap.config.path`.

See #36