Add CHANGELOG.md and README.md.

The extra-doc-files property requires cabal-version >= 1.18 (c.f.
http://coldwa.st/e/blog/2013-08-21-Cabal-1-18.html).  Bump this requirement
from 1.10.
Clean up cabal file.
Add homepage and bug-tracker to cabal file, linking to github.
Revert "User's guide (README)"

This reverts commit 06cc984b114e317d795c3e0c01b9be48b48b5724.
Revert "Default help page"

This reverts commit 74efe2d73c71b6459cd5f7d4719ef2500c87eeef.
Revert "Default front page"

This reverts commit 0d536654e146d0856ed9a959567f98b976b8dff9.
Bump version to 0.1.6.3 from 0.1.6.2.