README: tweaked examples, wording

the paths registered in the example hopefully convey that
the paths have no bearing on the resulting module/package
names.
basic python3 (3.4) compat

* make check PYTHON=python3 gives me

    PendingDeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses

  note: importlib seems to have been backported into 2.7,
  which means i should be able to abandon imp.

* `exec e1 in e2` is the same as `exec(e1, e2)` (2.7, 3.4)
* `raise T, args` is the same as `raise T(*args)`, the latter
  works in 3.x as well
tests don't need ellipsis that much

tighten the screw