Add email validation feature

[Bogardo]

Mailgun just launched a new email validation service called Guardpost which provides the following features:

• Syntax checks (RFC defined grammar)
• DNS validation
• Spell checks
• Email Service Provider (ESP) specific local-part grammar (if available).

Documentation:
http://documentation.mailgun.com/api-email-validation.html#email-validation

If think this could be a good addition to the Mailgun package.

Proposed syntax:

Mailgun::validate("foo@bar.com");

[CalamityJames]

+1, that would be very useful for my project!

[Bogardo]

I'll try to add this feature this week.

[Bogardo]

@CalamityJames Just added the new validation service. Please read the docs for more information