Makefile and Tileset Loading fixes

Added a few minor things to the .gitignore.

Fixed issue with makefile that caused `make install' to fail, as it was not copying over data/motd & data/credits recursively.

Fixed issue with makefile that was causing crash on attempt to load filedata.json when it hadn't been installed.

Fixed issue in cata_tiles.cpp where if PREFIX had been specified in make compilation, would still try to use current directory to find tilesets, resulting in errors.