Validate postcodes based on the country

Validation is based on wikipedia list of postal codes [1]. Formats were
converted into regexes for countries.

1: http://en.wikipedia.org/wiki/List_of_postal_codes

Fixes #753
Adjust handling of invalid forms

- Non-field errors render in separate alert blocks (including the
  exclamation icon)
- We also show a generic "there are errors" block at the top of an
  invalid form.