Modify premake script to generate visual studio files.

Currently the premake script only generates Linux Makefiles. While a hand-made visual studio solution is provided, it is intended that one should be generated by the premake script as this will ease maintenace in future. The projects in the examples directory also have their own premake scripts which need to generate visual studio solutions.

Currently the scripts take no options for setting system dependent variables such as paths to required headers and libs. Such options should be added also.