Add project root to less import paths

Adding this enables the ability to import relative to the project rather than relative to each individual file. See brief discussion [here](https://github.com/Nemo64/meteor-bootstrap/issues/28#issuecomment-68387702)

It may be interesting to expose this Less setting through a configuration file such as the [fourseven:scss](https://github.com/fourseven/meteor-scss/blob/master/plugin/compile-scss.js#L26) package does through an `scss.json` file in the project root folder.