calendar changes

Work in progress
Modifications to calendar.rst file.

Note to reviewers:  I don't see the "export button" or a way of opening
the "import dialog".  Please review that section carefully and provide
input.
Added *.snag files to .gitignore
syntax fixes & rename images - windows has case-insensitivity linux not
remove .snag file
Merge pull request #426 from owncloud/calendar.rst-changes

Calendar.rst changes