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.
The Python Installer provides the option to add python.exe to the path.  It is not enabled by default but the option is present (see png).

Adding `python` to the `%PATH%`  if not done during installation is not the most complex step, and likely within the ability of most users who intend to use `cask` on Windows.

Multiple versions of Python should result in the first one found on path being used.  Using a specific version (and not first found) would need adjusting `%PATH%` in the shell where it is being called from (or adjusting the `.bat` to explicitly call the desired version).  

![pythoninstaller](https://cloud.githubusercontent.com/assets/1301213/4217043/8a7c07be-38e2-11e4-9435-129d41800fee.png)