fix #186 ensure to be able to use `.` for username when invited

lib/form/invited.js

@@ -4,7 +4,7 @@ var form = require('express-form')
   , field = form.field;
 
 module.exports = form(
-  field('invitedForm.username').required().is(/^[\da-zA-Z\-_]+$/),
+  field('invitedForm.username').required().is(/^[\da-zA-Z\-_\.]+$/),
   field('invitedForm.name').required(),
   field('invitedForm.password').required().is(/^[\x20-\x7F]{6,}$/)
 );