Failing email validation "too-many-dots-in-local-part" test case added. Reproduce a bug that must be fixed
fix: now 'validate_email' return False if there are 2 or more '.' before '@'
Several 'validate_email' test cases added. 'consecutive_dots_in_domain_part' currently fail.
fix: 'validate_email' now return False if there is '..' in the domain part of the email address.
('consecutive_dots_in_domain_part' test now passes)
oops: forgotten to begin some test methods with 'test_' (weren't executed)
improved regexp used by 'validate_email' to don't match '.' at the beginning
and just before '@', wich was causing a unit test to fail.
fix: 'validate_email' now allows more than one '.' in the local part of an email address
(no '..' anyway), it's not a wrong syntax.

Verify it on GMail, for example.
email validation regular expression is now more verbose and self explaining