Normalize GitHub urls

fixes #7

builds on top of #78

ran this though all registered packages and it covers all of them except the following

```JSON
{
    "name": "ember-build-validations",
    "url": "git://github.com/dockyard/ember-builds/tree/master/validations"
},{
    "name": "grid-generator#1.0.1",
    "url": "git://github.com/martinmaricak/Grid-Generator/releases/tag/1.0.1"
},{
    "name": "kojs",
    "url": "git://github.com/SteveSanderson/knockout/blob/master/build/output/knockout-latest.js"
},{
    "name": "linq",
    "url": "git://github.com/mihaifm/linq/blob/master/linq.js"
},{
    "name": "RangeSlider",
    "url": "git://ghusse.github.com/jQRangeSlider"
},{
    "name": "squeezr",
    "url": "git://github.com/jkphl/squeezr/tree/master/squeezr"
}
```

All of which are invalid and can't be downloaded anyway.