introspect: Minor documentation fix
core: Add a per-stream flat-volumes flag

This is not expected to be used by most clients - the default
system-wide setting is preferred to provide a consistent user
experience.

However certain clients (at the moment, this is just web browsers), need
to be able to disable clients from modifying system-wide volume, so
this provides such a mechanism.
Add a .travis.yml for Travis CI

Currently uses Ubuntu's build dependencies, and runs make check and
check-daemon.