Changed "generation tx" to "coinbase tx" and added some HTML comments for me

1. Changed generation transaction to coinbase transaction, including
   removing a short paragraph about why generation txes are called
   coinbase txes.

2. Added a bunch of comments of the form DAH TODO. (DAH = my initials.)
   Some of these are comments for me to write or rewrite something; some
   of theme are just reminders to me to follow up on something.  Most or
   all of the will be deleted before my next planned pull request.

3. Corrected one minor grammatical error.
Merge branch 'dev' of https://github.com/saivann/bitcoin.org into dev
Merge remote-tracking branch 'tgeller/dev' into dev

Conflicts:
	en/developer-guide.md

Resolved conflit by accepting one of tgeller's conflicting changes,
postponing another confliting change by putting it in a comment, and
deleting the third which was attached to definition I deleted after
tgeller forked from saivann.
Merge remote-tracking branch 'saivann/dev' into dev

Conflicts:
	en/developer-guide.md
Merge pull request #9 from harding/dev

Global replace: "generation transaction" to "coinbase transaction"