Win32 dev adjustments

This PR is the work i did to be able to work with kart on my Windows 8 box. It does a few things:

1. It requires `os` to access paths in a more platform-general way
2. It adds some powershell scripts (`script/bootstrap.ps1` and `script/run.ps1`) for use in Windows (instead of using bash). It may help other Windows people get started more easily with this project. Typing in `script/bootstrap` into powershell will run the `.ps1` files instead of the bash automatically, so there shouldn't be a need to change the documentation.
3. Adds a windows specific grunt bootstrap task because i thought that might be clearer than doing platform-specific bootstrap tasks.

If you would like this to work differently, i don't mind adjusting it. This is what came to mind as a solution for me.

### Why i bypassed symlinking for running the app

i got rid of symlinking to run the app on Windows because symlinking requires admin privileges (which can be annoying on this Windows 8 machine, but manageable). When i ran as administrator, atom-shell didn't seem to work properly. i kept getting "not a valid atom-shell app", so i just ditched the symlink thing. i'll understand if that's not cool.

Thanks for this project. i'm having a lot of fun playing with it :smile: