Adds target `make uninstall`

Issue: #50
Moves common vars into cmake/variables.cmake

Issue: #50
Adds cmake module to find libreadline and libtinfo

Issue: #50
Adds cmake module to find LLVM

Issue: #50
Adds check for Threads in CMake script

Issue: #50
Adds toolchain for mingw32

Issue: #50
Adds FindPOD2MAN.cmake and FindGZIP.cmake

Adds option 'BUILD_DOCS'.
Now cmake is able to convert pod files into man pages.

Issue: #50
Moves targets 'doc' and 'image' into their own CMakeLists.txt

Renames get_pod2man_cmd into get_pod2man_cmd_wrapper.
Function get_pod2man_cmd_wrapper may produce a cmd which generates an empty doc (when arg STUB is set).

Issue: #50