Remove ghc-7.4 from the list of tested/recommended versions of GHC
(because of Hackage dependency problems).
Recommend version 7.10.3 of the Haskell Platform. (GHC 8.0.1 works too,
but I haven't tested the corresponding Haskell Platform packages.)
+ The instructions have been modernized and simplified slightly and should now
work again. (I have not verified it on Windows.)
Using 'cabal' instead of 'runghc Setup.hs' avoids some problems.
+ Also, download/index.html has simpler installation instructions sufficient
for many users, so mention it in the main README file.
+ Also adjusted the main Makefile.
+ Rename some txt2tags file from .txt to .t2t and remove abandoned .txt files.
+ Add program update_html that finds all .t2t documents and updates the
corresponding .html file. It can be invoked with 'make html'.
+ Add style to some .html documents
