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]