Merge tag 'v0.4' into develop

v0.4 released
Changed crossbar realms and ports (8080 → 9002)
Forgot to change port in pitt.js
Hotfix: Send pings only if there are peers connected
New docs: contributing guidelines
New docs: installation
Updated readme to include newer installation docs
New history in `README.rst`
Updated list of authors
Merge branch 'feature/history_authors_docs' into develop. Fix #24
Fix: don't "wanted to send ping" when no peers connected
Updated contributors list
WIP: application structure docs
Added "Design decisions" section to `STRUCTURE.rst`
New part of signalling docs.
Removing unnecessary colons
Changed beep volume from 50% to 10%.
Forcing video resolution
Description of signals and RPCs
Listed all events (RPCs and pubs/subs) happening in the application