Revert "Use Assembly.Load(Byte[]) instead of Assembly.LoadFrom."

This reverts commit b65fa30c7b21cce263ed9ada982bd47bc18eea04.
Look for an assembly that's already loaded before trying to load it from a file.
This fixes problems when trying to import pre-loaded assemblies that are embedded as resources.
Include directories from config var DESTDIRS in npython's runtime library dirs for python built with --enable-shared.
Merge pull request #50 from tonyroberts/develop

Fixes #48 and #49