API reference + checkhostname + symbol visibility fix

[bartbes]

I realise it's a bit weird to send a pull request for 3 things at once, and if needed I can split them up and send 3 requests instead. Anyway, here goes:

• I've created a small api reference, as that is something I couldn't find
• I've add a checkhostname function that checks whether the peer's certificate is valid for the given hostname, and made https.request check it. Note that without such a check, anyone with a valid certificate for anything could easily pull of a man-in-the-middle attack.
• I've changed the symbol visibility on linux so only the luasec api is exported

If there's any subset you want separated, give me a shout.

[bartbes]

Is there any progress on this? It's been almost a year at this point.