Fix problems with dylib linking

Changed postgres-dylib-linker shell script to ruby script, which uses otool -D to read library id:s from dylibs and changes them to @loader_path/../lib/library_id format with install_name_tool.
Then it uses otool to get linked libraries from dylib, so, bundle and executable files and changes
linking to @loader_path/../lib/ -format with install_name_tool

Linker option -headerpad_max_install_names is added to libjasper and libjpeg.

Scons build script python path is changed from /usr/local/bin/python to /usr/bin/env python
to make it work with system python.

In my tests this fixes issue #32 and most if not all dylib problems.