Move project definition to the top of CMakeLists.txt
Fixes #42.
Rename OpenDungeons.in to lower case
When packaging something system-wide for Linux, capitalized names are seldom used
Add proper "make install" instructions for Linux (WIP)
(and possibly Windows, though it's really basic for now).

Note that running the main executable in the build dir might be broken now, but we're not far from supporting both.