Returning wp_mail result after sending E-Mail
Return invalid E-Mail ID's in extract_email_ids() method too

Earlier it accept an array of E-Mail ID's and return only valid E-Mail
ID's from them. Now onwards it can also return invalid E-Mail ID's as
argument
Allow forwarding a thread to an E-Mail ID

It covers:
* A meta box in view thread page which allowes entering E-Mail ID to send E-Mails
* Sending thread replies along with its details to these E-Mail ID's
* Checking if user has permission and CSRF check
* Validating E-Mail ID's before sending
Add explainatory messages for user to failed ajax requests
Added action to wp_create_nonce()
Merge pull request #137 from VarunAgw/forward-conversation

Add the ability to forward a conversation.

Fixes #18.