Allow user to log emails without sending

Allow user to log an email activity without actually sending an email.

Use case:
User sees an email in their inbox.
They want to record both the underlying contact and information from the email in CiviCRM.

Add a "log an email" function.

Use Case from DGeilhufe
====================
Michal sent us an email about his contact with Jeff Waugh. I wanted
to log the email contact Michal had with Jeff in Jeff's contact
record in CiviCRM.

The interface didn't provide me a way to do that except via a custom
activity type or a note. I really just wanted to make it the same
activity type as if we sent him an email via our email functionality.
That way when I look at Jeff's email activities, I see all the email
communications with him.

If we need to make this type of logging into a custom activity type,
so be it, but it seems like people log "emails" in a shared contact
database, not "emails I sent this person". Especially if you can
figure out what emails you were involved with by pulling all the
email activities and looking at the to/from fields for your name.