Added load-preferences and preference-file-for-system/operation. Specialized
them so that preferences are found in ~/.asdf/<name-of-system>.lisp by default and are loaded on either a load-op or a load-source-op. Refactored load-op and load-source-op to both be subclasses of basic-load-op to facilitate this. Added test case in test directory. Still need to add to documentation of ASDF and will once I figure out where such a beast truely lives.