openssl

This commit is relatively major refactoring of the `openssl-sys` crate as well
as the `openssl` crate itself. The end goal here was to support OpenSSL 1.1.0,
and lots of other various tweaks happened along the way. The major new features
are:

* OpenSSL 1.1.0 is supported
* OpenSSL 0.9.8 is no longer supported (aka all OSX users by default)
* All FFI bindings are verified with the `ctest` crate (same way as the `libc`
  crate)
* CI matrixes are vastly expanded to include 32/64 of all platforms, more
  OpenSSL version coverage, as well as ARM coverage on Linux
* The `c_helpers` module is completely removed along with the `gcc` dependency.
* The `openssl-sys` build script was completely rewritten
  * Now uses `OPENSSL_DIR` to find the installation, not include/lib env vars.
  * Better error messages for mismatched versions.
  * Better error messages for failing to find OpenSSL on a platform (more can be
    done here)
  * Probing of OpenSSL build-time configuration to inform the API of the `*-sys`
    crate.
* Many Cargo features have been removed as they're now enabled by default.

As this is a breaking change to both the `openssl` and `openssl-sys` crates this
will necessitate a major version bump of both. There's still a few more API
questions remaining but let's hash that out on a PR!

Closes #452