Provide support for Windows (fixes `exec`)

The following changes provide support for Cask to run under Windows. (Possible fix for #199)

This will not provide support for `cask exec ecukes` or `cask exec ert-runner` because those applications are not supported under Windows (bash scripts).  It will however work with any application found on %PATH% or pointed to directly ( `cask exec "%programfiles%\\Microsoft Office\\Office15\\excel.exe" )

`cask.bat` acts as a wrapper to allow the script to be processed by python, without having to make any changes that will affect other OSes.

Regular expression is updated to include `\r?` to account for the CR.

Calls to `cask` from within itself or from `go` are updated to explicitly call `python cask` (same as in the .bat).  This allows Windows to process the script and should not have any effect on Linux.

I only experienced one issue when testing (using https://github.com/jleechpe/win-exec-test to update the URL to point to the correct branch).  Calling `cask` from eshell results in `Spawning child process: exec format error` but works without issue when called from CMD.  Copying the cask folder to another location (in my case `c:\bin\`) removes this error.  I had first tried renaming it to "~/cask" but the error still persisted.
@jleechpe I know about this option.  But the point is that it's off, and that this BAT file does **not** work with a *standard* Python setup, i.e. simply running the installer without any options.

Hence, this specific requirement needs to be documented.