Import is_email

Add is_email code and tests. Run test set with phpunit.
Update is_email tests

Small corrections on is_mail test set:

* bar.com has an A-record but not an MX-record

* test.com now has a MX-record

* xn--hxajbheg2az3al.xn--jxalpdlp does not have DNS records

Also include project status badges