Added README, COPYING, and algorithm-ideas.txt
Made combine_nodes not delete old data by default
Added test case for undo_move
Added test for combining moves
Implemented undo_move and combine_moves
Exhaustive search working (v. slowly)
Mostly working search with cache
Merge branch 'exhaustive-solve-with-cache' of https://github.com/splondike/pixel-painter into exhaustive-solve-with-cache