Use "e-mail" in all user facing strings

In the process of reviewing #361652: Document the project_issue schema in project_issue.install, I had to check the documentation style guide (http://drupal.org/node/338208) to see whether "email" or "e-mail" is recommended. I know that's for the d.o handbooks but it seems like a good idea to follow that, and at least in a few places I looked core also uses "e-mail". I did a quick look through the project_issue code base, and I see that we're inconsistent with what we use. In some places we have "e-mail" and others we use "email". This is in user facing strings wrapped in t().

Clearly this isn't a priority, but would be good to be consistent.