Merge app branch
This adds two commands for adding and removing 'apps':
0install add NAME URI
0install destroy NAME
Apps are a bit like aliases (and are intended to replace them). The main
difference between apps and aliases is that apps store their current selections
(in ~/.config/0install.net/apps/NAME/selections.xml). This means that they
start faster, because the solver isn't needed.
In the future, apps should also provide better desktop integration features and
make it easier to see what changed in an update.