Add rockspec file for installing from the repo directly

[mtourne]

I needed master rather than the version on luarocks to fix this issue :

https://github.com/wahern/luaossl/issues/53

Seemed easier to make the existing rockspec file work with the repo rather than figure out the build system

[mtourne]

ps. open to constructive criticism beyond a thumbs up / down (:

[daurnimator]

So far I've been maintaining the rockspec outside of the repo; I believe @wahern prefers to build+install via make.

I'd prefer we kept the status quo; we just need a release...

[mtourne]

My guess was that adding a separate option for installing as an added convenience could only be a plus, and never a downside.

[daurnimator]

Release is up. http://luarocks.org/modules/daurnimator/luaossl/20161029-0