Move cmd_(eval|function) to all objects of the ipc tree

They were in the manager object only previously, now they are in
CommandObject, which means all subclasses have it (including manager)

Now it's possible to use eval() from any part of qsh, for example:

    group['term']> eval("self.windows")
    (True, 'set([Window(st)])')

Or have keybindings that work on a specific kind of object:

    Key([mod], "x", lazy.window.function(lambda w: w.setOpacity(0.5))),

Figuring out any useful things to do with these is left as an exercise
for the reader.
Enable travis tests
Travis builds don't have SwapCached in meminfo
Add qtile Travis-ci status to README
Make Python 2.6 compatible

Dict comprehension and set literals are introduced in 2.7
Fix AttributeError recursion error