Email content history fix and additions

This addresses a bug in the email content history table where
"Unknown" was displayed in the number of emails sent column if any sort of
failure occurred during email sending. This behavior has been edited so now
the number of emails that failed to send is displayed, along with the number
of emails that were successfully sent. If the email task is still pending,
"0 sent" is displayed.

As a small addition, the table now also includes the authors of previously
sent emails, and the modal window for an email also displays its author.
I logged into the platform as  username:"sarina" and sent an email to myself. Here is what I see in my email client:
![screen shot 2014-08-11 at 11 36 51 am](https://cloud.githubusercontent.com/assets/1985317/3878215/67c9d1a6-216d-11e4-9b26-af7c7e069072.png)

It *does not* say the mail is "From: sarina". So I think using the term "From" is confusing, and I am scratching my head why you felt it was necessary to put in so many screenshots of your email client. What this table on the instructor dashboard will display is that "user: sarina" SENT the email. But "user: sarina" will NEVER appear in the "From" header on the email.