header: add If-Modified-Since support
Merge pull request #183 from cmr/if-modified-since

header: add If-Modified-Since support
Add missing reexport
Merge pull request #184 from cmr/if-modified-since

Add missing reexport
add etag header
Merge pull request #180 from wenderen/etag-header

add etag header
rustup for clone trait
Merge pull request #186 from hyperium/declone

rustup for clone trait
add allow header
Merge pull request #188 from wenderen/allow-header

Allow header

Closes #174
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.)