Add an option to build a debug version of Python

While you're writing C-extension, it is very helpful to use debug version Python. This patch enables you to build a debug version. The instruction to build a debug version is here: [Python Developer’s Guide](http://docs.python.org/devguide/). Just add an option `--with-pydebug`.

To avoid a conflict with existing non-debug version Python, it adds a suffix "-debug" to the install location. You can use Python "2.7.3" and "2.7.3-debug" simultaneously.

Could you review and merge this change?