(fix) Harden header parsing against memory exhaustion attacks.

Adds new limits on the lengths of header names and fields.

Fixes #187
fix(ssl): set_hostname on ssl connections
Merge pull request #191 from reem/max-header-lengths

(fix) Harden header parsing against memory exhaustion attacks.
Merge pull request #192 from hyperium/ssl-set-hostname

fix(ssl): set_hostname on ssl connections
rust upgrade
Merge pull request #195 from hyperium/rustup

rust upgrade
feat(client): add a new Client struct with super powers

- Includes ergonomic traits like IntoUrl and IntoBody, allowing easy
usage.
- Client can have a RedirectPolicy.
- Client can have a SslVerifier.

Updated benchmarks for client. (Disabled rust-http client bench since it
hangs.)