configure.ac: Added python autodetection feature.

Configure can now autodetect python version choosing from a wide range
of supported versions. If --with-python=force specified, then
configure fails if no suitable python version found. If --with-python=python-<version>
specified, configure fails if the specified version cannot be found.
Merge pull request #72 from talien/f/python-version-autodetect

configure.ac: Added python autodetection feature.