Combine GTK and HTML output into a single executable

The new --html output mode generates HTML and JavaScript for a web-site,
replacing the old precompute binary (which just generated some
intermediate files which could then be used to create the JavaScript).
Allow collecting from a Xen domain

Removed support for the old sexp format, as it complicated things.

Note that Xen support also has to be in a plugin, since it requires
libxenctrl.so, which might not be present.
Added Travis tests
Allow specifying Xen domains by name