Add C++11 thread pool class

This commit adds a class that provides a thread pool through C++11's
std::thread facilities.

Squashed commit of the following:

commit b1f20d254140ab1dc6877a1b2d107ad1797f1c09
Author: Peter Hille (png!das-system) <peter@das-system-networks.de>
Date:   Sat Aug 2 03:28:58 2014 +0200

    Add test code for ThreadPool class.

commit a4bc37d6abcb4f002e872896d92dd47a443c82eb
Author: Peter Hille (png!das-system) <peter@das-system-networks.de>
Date:   Sat Aug 2 03:28:20 2014 +0200

    Add C++11 thread pool class