Cleanup 3rd set of outstanding PRs

[jt-traub]

• Too few trade goods causes infinite loop
  • Pull in the work from PR No trade items bugfix #54
    • Avoid an infinite loop when we have less than 4 trade items.
• Clean up food item cost display in rules
  • Pull in work from PR Rules say no fractional food #120
    • Remove an extraneous comma
    • Add text saying partial food is lost.
  • Update rules snapshots with changes.
• Update rules to explain order of maintenance costs
  • Pulled in changes from PR Added clarification on how maintenance fee is paid #111
    • Added section to rules showing order of maintenance payments.
  • Updated snapshots to reflect changes.
• Fix longstanding economy bug
  • Pulled in changes from PR Update economy.cpp #8
  • economic improvement never counted users selling trade goods to a city/town
• Fix logical bug in world generation code
  • Pulls in changes from PR Fix using assignation instead of compare #53
  • All games except neworigins and standard had this bug - it had been previously fixed (partially) in those 2 codebases, but added parens to make sure comparison was correct.
• Small code quality cleanups on delete[] vs delete
  • Pulls changes for PR Use of non-array delete #16
  • While these didn't cause any errors/warnings during compile they are technically more correct code.
• Updated CREDITS file with some names that I was able to find that weren't already recorded.