Add feature to build luvi apps from lit

[creationix]

This feature would allow pointing lit to a package.lua file in a luvi app's source tree and build the binary including all dependencies.

> git clone git://github.com/luvit/luvit.git
# git clones down the luvit repo
> lit package luvit/app
# lit reads luvit/app/package.lua and downloads all dependencies
# lit combines dependencies with files on disk relative to package.lua into an in-memory zip
# lit appends zip to it's own binary half to create a new file in cwd.  Name is specified in package.lua
> ./luvit
# luvit is run from the newly created binary.

The app field in package.lua determines the generated binary's name.