Clean up links in Readme and Gemspec

Hi!

A number of links from imajes/geokit were redirecting to geokit/geokit so I pointed them directly to geokit/geokit.  A couple of sites no longer existed, so I removed links to them.

As a side note, http://geokit.rubyforge.org/ has some old, broken links, and an old version of them gem.  I'm not sure how much control you have over that, but given how high it ranks on Google compared to the github repo or rubygems.org, it would be good to either remove it or update it.