add a restcpp API backend

This commit offers an alternative to the dbus client to communicate with
a headless daemon. It uses restbed, a cpp framework for RESTful services.

This commits wraps up:
- The source of the restbed implementation (in bin/restcpp)
- The integration of the target to the configure.ac
- documentation
- Add external restbed library to contrib:
  - rules.mak file, to download the version 4.0, and compile it
  - Patch for ASIO, a dependency of restbed
  - Patch for the CMakeLists.txt file, to correct the installation path

[guillaume.roguez@savoirfairelinux.com: merged with Simon Zeni patch to support restbed into contrib]
[guillaume.roguez@savoirfairelinux.com: add tuleap ticket number]
[guillaume.roguez@savoirfairelinux.com: fixed configure.ac --without-restbed behavior]

Change-Id: Id49a1a04b05aac1e803981833abe5564785fb801
Tuleap: #498
Reviewed-by: Alexandre Lision <alexandre.lision@savoirfairelinux.com>
Signed-off-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>