Improve email validation (Issue #1)

Should follow the RFC 5322
(http://tools.ietf.org/html/rfc5322#section-3.4)

Return true for:
``` "John Doe" <john.doe@example.com> ```
``` John <john.doe@example.com> ```
``` <john.doe@example.com> ```
``` john.doe@example.com ```

Return false for:
``` John Doe <john.doe@example.com> ```
``` john.doe.@example.com ```
``` john.doe@example.com> ```
``` <john.doe@example.com ```
``` "John <john.doe@example.com> ```