Windows support

This Pull-Request adds support for Windows.

Pretty much everything works, except : 

- Streams are not supported, due to the fact that Tornado does not handle `add_handler` with non-socket fds on Windows
- gid, uid, rlimits and other Unix-specific stuff are not supported
- Daemonizing in `circusd` is not supported (but it could be though)
- Unix sockets are not supported

All the tests should pass, but I sometimes get some errors (like 1 over 5 times). It's essentially in `TestWatcher.test_unexisting`, when the process' status is `OTHER` instead of `UNEXISTING`.

As of today, I only got the chance to test it under a VM with Windows 7 and Python 3.4. I think it would be great to test it under other Windows environments.