Allow email body paragraphs to begin with 'On'

One of the `EmailParser` regex seems a bit greedy. I'm unable to begin paragraphs with the word 'On' because they get stripped out by the parser.

This body currently returns an empty string:

```
On the counter.

On Tue, Sep 30, 2014 at 9:13 PM Tristan <email@example.com> wrote:
> Where's that report?
>
> Thanks, Tristen
```

The change in this PR fixes this, but makes is so that bodies like the example below would not get split:

```
Hello.

On Tue,

Sep 30, 2014 at 9:13 PM Tristan <email@example.com> wrote:
> Where's that report?
>
> Thanks, Tristen
```

We don't have a test for the example above, so I assume it's not important. I've never seen an email body with a blank line in the middle of "On...".

cc @r00k because he's interested in this fix.