Edits To DevGuide

I just re-read the entire DevGuide with my editor hat on and made a
bunch of small edits.  Here are the highlights:

* Removed a couple paragraphs from the block chain section which were
  outdated. One paragraph described things that have been moved
  elsewhere; the other paragraph described transaction fee details that
  are now better described in the Transactions section.

* The micropayment channel contract has been updated to use the terms
  "bond transaction" and "refund transaction" instead of the much less
  useful "first transaction" and "second transaction". The micropay
  image was updated similarly (also using fontname=Sans and optipng).

* I removed some unnecessary instances of millibits. I think the only
  millibits left correspond to images that use millibits.

* I deleted some old and now-unnecessary comments

* A bunch of minor grammar errors fixed, plus a bunch of small one-word
  or a-few-words changes that I think improve readability.
Fix Confusion Found By @saivann (thanks!)
Merge pull request #120 from harding/edit2

Edits To DevGuide