Generate help and man page from README.md

README.md usage and key bindings updated from source code.
Both removed from source, inserted via generated include files help.h and keys.h.

README.md headings changed to enable man page generation.
Man page generated via http://mantastic.herokuapp.com
Much simpler than a dependency on a local ronn with Ruby-based stack.

Man page related cleanup in Makefile and native toolchain setup script.
First rule dist, to avoid omxplayer-dist folder becoming root-owned,
caused by dist being a prerequisite to install.

README.md now has a warning about itself being a source file.