Standard header

Classes for standard headers defined in the RFC.
E.g., instead of "new Header('Host', hostname + ':' + port.toString())", use "new HostHeader(hostname, port)". It's less error-prone and easier to read.
This is the first step in defining header classes. There's only HostHeader so far, but the goal is to have a class for every header defined in the RFC. we just need this pull request now to combine the rename, changes to docs, and improvements to HeaderCollection.