Renaming the workshop creation script for upload to Software Carpentry website.

This PR renames `tools/create` to `tools/swc-workshop-create` so that when we upload it to http://files.software-carpentry.org, and people download it from there, its purpose will be clearer --- `create` on its own isn't particularly meaningful.

This PR also does some minor copy-editing on messages inside the script.
Merge pull request #90 from gvwilson/renaming-creation-script

Renaming the workshop creation script for upload to Software Carpentry website.