Move all sources into src

Also, there's not really a reason to have main.lua loaded as a header.
It's just as easy to compile it as a normal lua object file and tell
main.c to require it directly.
Update stubs to actually, you know, work
Create a location for examples and move them there

Also, this adds more SGF examples from the official SGF site as well as
the C sources for a SGF syntax checker. This will be useful for writing
a SGF parser in Lua.