Add rockspec file

This pull request integrate typeslua within luarocks. You would need to:

- change the repository url in the rockspec file to point to your repository after you merge this pull request
- upload the luarock to the luarocks servers

There is something I'm not too satisfied about yet. I modified the way the `lsl.tld` file is loaded. Using a Makefile, I generated a `lsl.lua` from the `lsl.tld` file, and load the types from there.

This is because I can only include lua files in the rock (or I don't know any other way) and also because when installed, knowing the path where the file might be located would be tricky. Using a lua module, finding the file is already taken care of.

I used the builtin builder in the rockspec instead of the make builder because finding the correct installation paths using the make builder might be complicated as well. I have a rockspec installation that is not in a standard location (such as /usr/local) and the make builder don't work in that case (or I don't know why).

Note: this is the first time I wrote a rockspec so I might have missed a few things.

Mildred