support for relative paths on sequelize.import

Thanks for all the work on sequelize!

This patch makes this possible:

var Project = this.sequelize.import("assets/project")
or
var Project = this.sequelize.import("./assets/project")
where only
var Project = this.sequelize.import(__dirname + "/assets/project")
would work, previously (and still does).

I hope this contribution is usefull