feat(ssl): make OpenSSL an optional dependency

[nstoddard]

Compiling with default_features=false will now omit SSL support.

This introduces a ton of cfg annotations, but I don't know how to avoid them :(. It's quite ugly, but it seems to work.

BTW, I hope I got the title format correct.

Fixes #541

[coveralls]

Coverage remained the same at 85.64% when pulling fcda403 on nstoddard:master into 5747792 on hyperium:master.

[pyfisch]

openssl is optional for cookies: https://github.com/alexcrichton/cookie-rs/blob/595a9e095bef8d98691bd1eed135d9e540f279e0/Cargo.toml, so you should not kick out cookie support completly.

[pyfisch]

* imports are evil, so do not change the normal import.

EDIT: In this special case they can stay.

[seanmonstar]

I agree, let's not remove cookie entirely. We can turn just turn on or off cookie's usage of openssl.

I seem room for improvement in cookie's feature usage though: the jar could still be provided, and just have the signing methods and types conditional inside jar.rs, but oh well.

[seanmonstar]

Besides those comments, this looks great! I'd been thinking of providing this at some point, but never got around to it. Thanks for the effort!

[mlalic]

Since the HTTP/2 stuff got merged, openssl found its way back in :) I've published a new version of solicit (0.3) to crates.io, that relegates it to an optional dependency too, behind the "tls" feature. Hyper doesn't use anything from there yet anyway, so it should be fine to just go with the default set in all cases.

[seanmonstar]

This was actually merged in #577.

[nstoddard]

Oh, I totally forgot about this PR. Sorry about that!