Change expression constants names to the ones of equivalent commands

Expression constants now have the same name as equivalent commands (e.g.
"sp").
General cleanup
Improve double expression constant look

* Double expression constants are rounded to a sensible decimal place.
* Double expression constant values which do not have decimal places no
longer show a decimal point.
Update changelog
Add more information to README.md

Add a link to the wiki page and some precompiled build information to
the README.md file.
Generalize JDK download link in README.md

The JDK download link, which is located in the README.md file, now
points to an overview page instead of a download page for a specific JDK
version.