Add Email::HtmlCleaner for email processing
Start splitting up methods in Receiver
TODO - write a good commit message

Receiver spec changes (squash)

Change boundary.eml - same site title

Remove multipart.eml as it didn't make sense

I don't know of any email client that puts the reply in one part and the
original in another part - that seems a bit crazy. The test only worked
because we were choosing the plain-text first.

Rewrite receiver tests

Add InvalidPost tests to Receiver

Give attachment.eml a REAL test

Receiver spec changes (squash)