update instructions

refs #2454
use fdroidrepo:// URI in QRCode so that Android will direct it to F-Droid

There is a merge request in to make F-Droid register to receive all
fdroidrepo:// and fdroidrepos:// URIs.  This comment makes the QR Code use
that URI so that it'll be automatically directed to F-Droid when scanned.

Here's the related merge request:
https://gitorious.org/f-droid/fdroidclient/merge_requests/39

refs #2470
ignore files generated by 'android update project'
for nanohttpd, switch from jars to git submodule, we need the latest

Looks like there are some key bug fixes since the last jar release,
and SSL support got included in master.