Updated todo.
Added "LightGray" as a color.
Added commands_listing
Added new sources to .bib-file.

Added dlvhex, clasp, gnt, platypus as DLV solvers.
Added Liftschitz's 2008 "What is anser set programing?" and Baral's 2003
book.
Corrected misspelled property in lotze1999.
Added commands_listing include to main file.
Began describing CFP and DM.