Key Headers HashMap with SendStr instead of &'static str

This fixes an unsafe transmute that could cause use-after-frees
on collision resolution in the headers hash and avoids allocating
for cases where it is not necessary.

Fixes #12
Merge pull request #14 from reem/string-headers

Key Headers HashMap with SendStr instead of &'static str
docs typos
todos
fix benches/client with new Handler
abstract out NetworkStreams
use NetworkStream in client