Support WinRM communicator, offline builds, more

### Enhancements

* Update to work with WinRM communicator
  (https://github.com/mitchellh/packer/pull/1175)
* Support offline builds by searching for files locally, before
  attempting to download them
* Strengthen error checking/reporting
* Add `floppy/unzip.vbs` to remove 7zip requirement
* Add `lessmsi` to avoid *installing* 7zip to extract files from iso
* Prepare for https://github.com/mitchellh/packer/issues/1181
* Move most cleanup/delete code to `clean.bat` to aid debugging
* Download VMWare Tools ISO, if not available locally
* Download Virtualbox Guest Additions ISO, if not available locally
* Add `CYGWIN_MIRROR_URL` to `_packer_config.cmd`. Fixes #12
* Add 5 other useful variables to `_packer_config.cmd`
* Add `_packer_config_local.cmd` (similar to `Makefile.local`)
* Allow many script variables to be overriden in `_packer_config*.cmd`
* Standardize scripts to use same logic, and case, throughout

### Fixes

* Wait until ssh service is stopped before opening port 22
* Wait until winrm service is stopped before opening port 5985
* Wait until installation is done before starting winrm service
* Replace `timeout` with `choice`, as `timeout` sees mouse click as
  keypress in `00-run-all-scripts.cmd`
* Fix `_download.cmd` to work if filename differs from URL
* Uppercase environment variable names in `.ssh/environment` to be
  consistent with Cygwin standard
* Replace `%TEMP%` with `%SystemRoot%\Temp` in `openssh.bat`