[eigen] Lua wrapper

Hi!

I've been hooking Eigen up to Lua: https://github.com/ggcrunchy/eigen-lua

Primarily I'm using this as a Corona SDK plugin (a fairly recent binary + example for Windows may be found here: https://forums.coronalabs.com/topic/69079-wip-for-eigen-plugin-number-crunching/), but the SDK dependencies are rather few and minor in nature.

The only other binding I found didn't seem to be too far along, nor updating. This is maybe a month and a half along. I'm taking a break for a few days, so I figured I should finally mention its existence. :) Although I've been developing it separately, at the moment the public repo is current.

Parts in flux: Maps seem to work off and on as I add features. Off at the moment. :( Views are maybe halfway there. Vectors are rather shoehorned in right now. It's still pretty arbitrary which types get specialized in operators, e.g. multiplication, versus being copied to a temporary first.

I'm aiming for a reasonably complete binding of the dense API, and maybe geometry shortly thereafter. I'm on the fence about doing the sparse API any time soon.

I've been using this for the QR / LU solver, and have various situations where blocks would be an ideal fit that I mean to try, and some other scenarios that I forget. Otherwise, I'll probably try to port a bunch of examples to start testing features. Who knows how long this will take. :)

Anyhow, I'm probably omitting important things, but I'll leave it at that.

Cheers.

- Steve