configure uses python2-config

i have an archlinux box with python-{2.7,3.4} installed (including
development files), and the python bindings for libstorage failed
to build because neither python package puts Python.h where
libstorage expects it.

double-checked on openSUSE-13.1.
Merge pull request #72 from roman-neuhauser/python2-config

configure uses python2-config