v3.0.0 release

[3.0.0] - 2018-11-26

Added

• Added overloading constructor which deducts number of available thread from
  the hardware.
• All code now lies in the :code:ThreadPool namespace.
• Add an option to either download dependencies or use system wide install.
• Added work stealing in the ThreadPool to increase performance.
• Add multiple macros to select whether or not you wish to enable work stealing.

Changed

• ThreadPool implementation is now using multiple queues with multiple workers.
• Remove hook copy overload (it was wrong)
• Changed dependency management (gtest/gbench) to be downloaded at compile time
  and not at configure time.

Removed

• The ThreadPool is now a fixed size pool. I removed the ability to adapt the
  number of threads at runtime.