Version 0.7.0 Beta Release

This release has some breaking changes for users of properties such as NNG_OPT_RAW and NNG_OPT_PAIR1_POLY, as well as users of the ZeroTier transport.

Most of the other changes in this release amount to massive documentation improvements.

However, Linux now uses epoll(), thanks to a contribution by Liam Staskawicz, which should lead to greater scalability.

There is a new nng_sleep_aio() API, and a demo/async program that demonstrates using the async AIO features.

Please also note that the documentation on the website and on Leanpub has all been updated to reflect this.

We are getting much closer to a 1.0 release, so I'm really grateful for testing and review feedback!

Thanks!