zlib update to 1.2.8 and cleanup 

* after the inclusion of the zlib sources into the project some files are doubled
* zlib was outdated (1.2.7), now it is updated to 1.2.8
* updated README to not generate a zlibstatic.lib

The integrated zlib was not working for me together with Qt 5.1.1.
After I upgraded my Qt to the latest 5.3.2, it is working.
So it should be noted, that older Qt versions may generate problems.