Feature tests around sign-up and confirmation

[corajr]

Urgently needed to ensure that people can actually sign up.

Tests:

• The sign-up process
  • [x] keeps the user's info (esp. gender; see issue #20)
  • [x] displays any errors in the form
  • [x] redirects to /users/sign_in (with mention of the confirmation email) when successful

[corajr]

Resolved by 63e86a2.