Absolute script uri's

This is my first time working with Jam, and I am fairly new with require, so sorry if this is a totally noob problem/non-problem.

I was hoping to roll Jam it into an Express.js app I am working on.  Maybe there is something for this that I am totally missing, but I saw #84 and think this would solve both problems.  My `package.json` looked like this:

    "jam": {
        "packageDir": "public/js/lib",
        "baseUrl": "public/js",
        "dependencies": {
            "jquery": "1.8.3"
        }
    }

After building the require config files, this is what they looked like:

    require.config({
        "packages": [
            {   
                "name": "jquery",
                "location": "lib/jquery",
                "main": "dist/jquery.js"
            }   
        ],  
        "shim": {}
    });

Notice the `"location" : "lib/jquery"`.  What I wanted this to be was `/js/lib/jquery`.

This change adds an option `locationBase` (might not be the best name...), which is then appended to the location, resulting in:

    // package.json
    "jam": {
        "packageDir": "public/js/lib",
        "baseUrl": "public/js",
        "locationBase": "/js",
        "dependencies": {
            "jquery": "1.8.3"
        }
    }
    
    // require.js
    require.config({
        "packages": [
            {   
                "name": "jquery",
                "location": "/js/lib/jquery",
                "main": "dist/jquery.js"
            }   
        ],  
        "shim": {}
    });

I setup the tests and ran them after making the change and only got one failure:

![jam-tests](https://f.cloud.github.com/assets/1027776/29374/562a499a-4d6a-11e2-973d-6d21f161c6bf.png)

I dont think that has anything to do with the changes, but let me know if I am wrong...

Anyway, good work on the package manager!!