Contrib: remove outdated slackbuild script

The script is out of date and lags behind the current version available at
http://slackbuilds.org. Added slackbuild pointers to README and web page.
Contrib: remove obsolete ipgk build script

The format has been deprecated in favour of opkg, see
http://en.wikipedia.org/wiki/Ipkg
New release 0.7.6

This includes a fix visible on ncurses >= 5.9.20130504 (currently
on Slackware 14.1 and Debian jessie). Please see NEWS for details.