Support newest python.el for elpy-use-ipython.

Emacs 24.4 will ship with a newer python.el which can correctly
detect and interact with IPython. Support this new mode when available
and don't set all the deprecated values for it.
Handle bad directories in sys.path.

Python 2.7 fixed a bug in 2.6 where directories in sys.path without
read permission would cause spurious exceptions. Elpy now deals with
these errors. At least as long as we officially support 2.6.

Fixes #275.