Moved loading code into load_ruby().
Adjusted RUBY_SRC_DIR for rubinius.
Default RUBY_ARCHIVE and RUBY_SRC_DIR in load_ruby().
Not supporting --skip-update for now.
Updated the README.
Don't use $SRC_DIR in output messages.
Move auto-detection into individual functions and remove auto_detect.

* This should improve testability of the core functions.
Moved functions which the build scripts can override into functions.sh.