Ref #15 - Read command

- Modified the list of default commands to include 'bye', taking it away
  from the start prompt. 'bye' isn't actually a method, but it does exit
  observa.
- Changed the prompt from ':' to '>', because...well, this is Ruby and
  that's what irb uses, and I guess it just sort of feels right. Totally
  personal preference.
- Added a 'read' command, which calls Kernel#load. The reason it's
  called read instead of load is because we're supposed to be pretending
  as if we're asking an independently thinking piece of (albeit dumb)
  software to go read something in order to get smarter.
- Added a couple of extra spaces in front of each command in 'list'
  because it seems a little nicer to read.

So...rather than the only useful addition (the new 'read' command), it's
mostly little clean up things.