Build html5 cache manifest

First try at auto build of app cache manifest

To test just add a package like this to your slug.json packages;
```json
 "cachemanifest": {
      "paths": "./public",
      "target" : "./public/myapp.appcache"
    }
```

All the files found under paths will be added to the the target. The header will be the current timestamp. With this browsers will discard the cached version and pick up the new.

###### TODOS and issues:
- [ ] Test it a bit better
- [ ] Ignore hidden files such as osx ds_store