OS X (and theoretically BSD) compatibility

Three minor changes:
1. Install script creates `/usr/local/src` if it doesn't already exist
2. OAuth looks for `md5` (BSD default) if `md5sum` (GNU default) doesn't exist
3. Rewrote the README to clean up phrasing

This should allow the script to work on OS X out of the box, as `md5` is a default command from the bundled OpenSSL distribution. It is also a BSD standard, and should work on anything else. It's no longer necessary to install a package from homebrew or macports that does the exact same thing that a builtin does.