Fixed user POST request: now it doesn't allow the registration of an already
existing (either active or inactive) user.
Removed test_repeated_post which used to allow a new user to register itself 3 times.
Insted, added test_post which tests a new user creation.
Added username control: it has to be a valid email address.
Added new function validate_email that checks through a regular expression the email validity.
Added test_user_creation_with_invalid_email to test this.
Merge pull request #70 from Roxana28/fix_pendingusers_issues

Fix pendingusers issues