Add lookupName function.

[cmears]

Example of use:

-- If a straight line was drawn between two named subdiagrams, but
-- stopped at the edge of each subdiagram, where would the endpoints be?
connectionPoints n1 n2 d = do
  b1 <- lookupName n1 d
  b2 <- lookupName n2 d
  let v = location b2 .-. location b1
      midpoint = location b1 .+^ (v/2)
      p1 = fromJust $ traceP midpoint (-v) b1
      p2 = fromJust $ traceP midpoint v b2
  return (p1,p2)

(We also need to export it in diagrams-lib.)

[byorgey]

Ah, please don't add stuff to the top of CHANGES.markdown -- it screws up my automated scripts for making minor (bug fix-level) releases, causes merge conflicts with the stable release branch, etc. Perhaps instead we should make a file called CHANGES-HEAD.markdown or something like that, which can be added to the top of CHANGES.markdown when the time comes to make a new feature release?

[byorgey]

Looks good, other than my comment re: CHANGES.markdown.