rename polygon_collision_detection.hpp/.cpp to geometry.hpp/.cpp

It handles more than just polygons these days.
fix polygon_visualizer for units
fix bounds-checked-ints
specific_object_type_implementations.cpp -> specific_object_types.cpp

It was just implementations for its header file, and ".cpp" itself
implies implementations.
Give text files a .txt extension.

Now everyone will will think we're Windows developers ;-) ;-) ;-)
-- but seriously, it improves clarity a little bit
and usability for those with OSes that auto-open files
based on their extension.
code update: use lint* synonyms for lasercake_int everywhere

(merely code style: the lint typedefs are newer and textually shorter)
rename polygon_int_type to geometry_int_type
Make gl_data_preparation.hpp not depend on GL headers.

This makes the header be able to compile on its own,
so IDEs and documentation tools (and people) are less confused.
zbox_visualizer/README: wrap lines to 80 characters
give zbox_visualizer/README an extension

.txt would be fine; .markdown allows github to render it more
fancily.
convert INSTALL to markdown so it looks better on Github
Convert principles.txt to markdown and update it somewhat.
convert DEVELOPER_README to markdown
wrap some very long lines
rename tile_physics_sub_tile_units -> tile_physics_sub_tile_distance_units

The latter is more correct.
rename another README to .txt
update INSTALL information
dev-build.py: use a smarter default number of CPUs.