Graceful failure for cask link

Currently cask link fails with a "Wrong type argument: stringp, nil"
if cask is called without arguments. This fix prints the help doc
instead.
Link and exec documentation updated.
Add test for using link command without argument
Merge branch 'fix/error-handling-on-link'
Merge branch 'fix/link-exec-documentation'
Release v0.7.2.
[Fix #238] Handle missing Emacs more gracefully
Handle OS errors more gracefully [#238]
Refine error messages [#238]
Update go to support Windows

Explicitly call `python` to ensure script is properly parsed in Windows
Provide `exec` support for cask under Windows

Explicitly call `python` in `get_cask_path` to ensure that the script is parsed under Windows.

Also include `\r?` in VERSION_RE to account for CR in output under Windows.
Create cask.bat wrapper for cask

Calling `cask` from the command line will match `cask.bat` under Windows.  This script will explicitly call `python cask` and match the python script (sans-extension).
Change python to sys.executable

As per lunaryorn's request.
Change python to sys.executable

As per lunaryorn's request.
Remove \r? and use universal_newlines argument

Removed \r? from VERSION_RE and instead use universal_newlines=true in
subprocess.Popen
Update Installation with Windows specific

Added Windows specific installation instructions on how to add `python`,
`emacs`, and `cask` to %PATH%.

Provide both command line and gui methods to update %PATH%.
Fix typo in Environment Variables
Typo Windowx -> Windows
Punctuation and :guilabel:

Update ``{var}`` to :guilabel:`{var}` where appropriate.  Also fix one
``Path`` to :envar:`Path`.

Added missing punctuation on sentences.
Merge pull request #239 from jleechpe/win-exec

Provide support for Windows (fixes `exec`)