add attachment conversion support

Hello,

this patch adds support for converting attachments. It posts them as gists (one gist per attachment) and then posts comments to the ticket.

Limitations:
* it fetches attachments over HTTP (I don't recall whether they're in the trac DB too)
* the comments linking to the attachments appear before all other comments, instead of chronologically like in Trac

When I used this, I hardcoded the URL. For this pull request, I moved the URL to a variable. I did not test this.

I extracted this patch from a bigger bunch of changes; I did not test the code after extraction. If anything is broken, my apologies.