Produced an SVG variant of Nico's happy-path .pptx

The nico-20161122 branch contains a powerpoint file and its generated
.png output.

Autocrypt is an open standard and should be documented with open
standards that are editable with free software, so i reinterpreted the
graphic as an SVG.

Also, it should be possible to embed or link to the .svg in the
generated html documentation without needing to convert formats.

In the course of re-interpretation, i made a few specific changes:

 * Font normalization: the image now depends solely on Arial and
   Courier New, which should be available everywhere.

 * Alice and Bob's avatars are derived from the freely-licensed Tango
   iconset.

 * Raw e-mail message contents are a consistent font and size, as they
   tend to be presented in any "message source" view.

Ideally, we want this included at the top of the "1:1 happy path"
section of doc/examples.rst but i'm not sure how to do that.