Created `make install`

Created `make install` rule, which installs to /usr/bin and
/usr/share/gtktactoe/sprites.

Added code to makefile and gtktactoe.c to make gtktactoe load its
sprites from the installed location, as set by the PREFIX and
PREFIX_QUOTED variables in Makefile.
Updated README

Updated README with information regarding paths and the new `make
install` command.