Listener: requestLoad does not return a resolved promise anymore but instead directly returns the result

Async: implemented a whenAny primitive that returns a Promise that will be resolved when one of the Promise is resolved

Async: added a way to rethrow an exception and then propagate the exception along to an other promise

Do not call Request::associatePeer anymore until the mysterious lock contention has been found and resolved

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