Basic user authentication module

For #281. I tried to get this as simple and as decoupled as I could to allow for customization on the app side.

This is aimed at apps with stored user information that either use `auth.User` or embeds it in another struct.

## Usage

    package app

    import (
    	"github.com/robfig/revel"
    	"github.com/slogsdon/acvte/app/controllers"
    	"github.com/slogsdon/acvte/modules/db"
    	"github.com/robfig/revel/modules/auth"
    )
    
    func init() {
    	// Filters is the default set of global filters.
    	revel.Filters = []revel.Filter{
    		revel.PanicFilter,
    		revel.RouterFilter,
    		revel.FilterConfiguringFilter,
    		revel.ParamsFilter,
    		revel.SessionFilter,
    		revel.FlashFilter,
    		revel.ValidationFilter,
    		revel.I18nFilter,
    		revel.InterceptorFilter,
    		revel.ActionInvoker,
    	}
        // tell auth module how to grab our user
    	auth.GetHash = func(username string) *auth.User {
    		user := new(auth.User)
    		db.Db.WhereEqual("username", username).Limit(1).Find(user)
    		return user
    	}
    	// the Admin controller requires authentication
    	revel.InterceptFunc(auth.CheckSession, revel.BEFORE, &controllers.Admin{})
    }