Basically, this commit brings a brand new Response API to the party:
   - Response::send() can be used to send a fixed-size content and
     will use the classic Content-Length header
   - When content size is not known in advance, a Stream interface can
     be used to stream the response. A ResponseStream is obtaining by a
     call to the Response::stream member function. After that, headers
     will be immutable and data can be streamed a-la-ostream with
     operator<<

Also fixed a bunch of issues:
    - Flags<T> now works correctly and correctly zero-initializes the
      value in the default constructor
    - Fixed a nasty bug in Poller that went unnoticed because the Tag
      constructor was not marked explicit

An Http Response is now written in two phases:
   - First the headers are written, and then a ResponseWriter is
     returned once all headers have been written.
   - A ResponseWriter is basically a wrapper around a NetworkStream
     which is a streambuf, allowing it to be used with an ostream object
   - Once a ResponseWriter has been constructed, headers become
     read-only and can not be written anymore

The whenAll function returns a Promise<T> that will resolve
when all of the promises have resolved or will reject when of
the promise is rejected.

async: got rid of code duplication. Removed the Promise<void> specialization and specialized the Core instead

commit e60bcb0bc15fbba453340d69d106c5850b9a6d67
Author: octal <mathieu.stefani@supinfo.com>
Date:   Thu Nov 12 14:14:53 2015 +0100

    Added a NoExcept concept that will call std::terminate if a promise has been rejected

commit bae200046427b93e6f124c848ab36d246a7580c3
Author: octal <mathieu.stefani@supinfo.com>
Date:   Wed Nov 11 14:50:27 2015 +0100

    Updated Optional to be able to move the result out when chaining a Promise

commit da60bc8b02a1fb89a91985a0da9ddf4b7219f5b3
Author: octal <mathieu.stefani@supinfo.com>
Date:   Fri Nov 6 17:43:36 2015 +0100

    Started working on a proof-of-concept implementation of an asynchronous primitive based on the Promises/A+ specification

instead is a shared_ptr

Conflicts:
	src/http.cc

Added a Date and a Raw header. Raw headers will be used when the parser encounters a header that it does not know. Raw headers only store strings for now

The reason is to avoid some headers name like Server or Date to collide
with some other generic classes that might have the same name