GlutHost will run "main.lua" if host binary is called without arguments
add Lua global 'arg' that mimics the lua/luajit built-in global 'arg' to GlutHost

The host's argv is converted to the lua global 'arg'.
This allows the moai binary to be a drop-in replacement for
lua or luajit in the Busted (like Rspec) test framework.

See AKUSetArgv() for arg array order.
Merge pull request #634 from robertwahler/add_argv_global

Add Lua argv global to Glut host, run 'main.lua' if host given no args