fix a typo.
Allow the show function of click exceptions to work without context.
Merge pull request #71 from Christophe31/patch-1

fix a typo.
Merge pull request #70 from julen/doc-fixes

Docs: minor fixes
Added a helper method to add help options and refactored the group decorator a bit.
Added strong support for a defined parameter callback evaluation order.
Docs: fixes