Icemon 3.0.1 release

Exactly one year after the 3.0 release, here's the next patch release of the Icemon 3.x series.

No major changes this time, merely bug fixes and small code refactorings.

Here's the changelog for the 3.0.1 release:

Bug fixes:

- Added work-around for build for icecc.a using old CXXABI (#24)
- Fixed build with Qt 5.5 
- Improved how docbook2man is searched for (PR #21)

Internal Changes:

- Added Doxygen support to CMake
- Modernized CMake code (FindIcecream.cmake, etc.)
- Modernized source code to use C++11 features (override, nullptr, auto)

Get it:

$ git clone https://github.com/icecc/icemon 
$ git checkout v3.0.1

Enjoy!