Support for using Rubber from a local Windows machine

(Remote machines must still be Linux)

- Add local OS detection (Windows, Posix) to BoundEnv class. (To add support for more platforms, see: http://rbjl.net/35-how-to-properly-check-for-your-ruby-interpreter-version-and-os)

- Add ability to write local hosts file in Windows format . Requires to run cmd prompt as admin.

- Writing to etc/hosts and known_hosts files now throw an error if user does not have write access.

- Add 'local_windows' Vulcanizer, including:
     * fixes a known limitation in Bundler when copying/deploying code cross-platform.
     * adds remote library dos2unix
     * add 'use_dos2unix' variable

- If 'use_dos2unix' is set, remote calls 'dos2unix' after code and/or config transfer.