make Outline::Lua compile and work on Perl 5.12+

[maddingue]

The proposed changes allow Outline::Lua to compile on recent Perl, as well as making sure the module actually works on these versions.
Tested with 5.12.4, 5.14.2, 5.14.3, 5.16.2

[Altreus]

@maddingue Hey - I rebased your contribution onto my local branch so I could have a look at the diff.

Yours passes the tests and is much simpler than mine so I reckon that means it's better :)

I suspect you are more experienced in this than me; I would greatly appreciate some guidance of what in my own version I did not need to do, e.g. what of my version is just repeating myself, what I've misunderstood, etc. It seems like what I originally had has changed much less than I think it has.

Here's my version: https://github.com/Altreus/outline-lua/blob/80a5c5d/Lua.xs#L105

Anyway your version is current master, and since I rebased I'll close this :) And thanks!

11dd328