Create TESTING.HOWTO.md

adding some testing information for users testing stuff
more robust email regex
- matches asdf-asdf+asdf3@asdf.asdf.com
- fixes #259
refactor hardcoded prereg sold out / not yet open / closed text back out to templates
added magstock-specific versions of this text (it's probably not super-needed)
closes #215
actually test this before I check it in, fix filename path
Merge pull request #268 from EliAndrewC/refactor_prereg_closed_txt

refactor prereg text for closed/open/sold out into external templates
setup magstock-specific versions of those templates
Merge branch 'master' into develop
Merge branch 'bug_email_regex' into develop