Revise the text to not recommend the CC licenses for software

 - Creative Commons licenses are good for manuscripts and
   presentations but shouldn't be used for software.  In particular,
   they are not compatible with GPL.
Add link to Jeff Atwood's excellent blog post re software licenses.
Add missing http://
Merge pull request #4 from kbroman/master

A few edits regarding software licenses, most importantly to recommend _against_ CC for software