depends: Add a package for qt4.6. Linux uses it by default.

We're not ready to switch to a static qt5 for Linux yet due to missing plugin
support. This adds a recipe for building a shared qt4 that we build and link
against, but don't distribute.
gitian: descriptors overhaul

Descriptors now make use of the dependencies builder, so results are cached.
A very new version (>= 3a0e9024b) of Gitian should be used in order to take
advantage of caching.
quick docs update
travis: let travis use shared libs for tests