EmailValidator: Standardize messages.

Instead of having "not valid" and "invalid" in different messages, we just use "invalid" everywhere.
EmailValidator: Strict e-mail checking.

Invalid e-mail addresses such as "foo@bar.com((((" would be valid because of the regexp.
EmailValidator: Fix failing tests.
Merge pull request #248 from duckduckgo/jag/improvement/email-fixes

Fix e-mail validation problems.