The demo relied on the build context returning accumulated times in
microseconds. On Windows this was using QueryPerformanceCounter directly
which has a higher resolution, thus returning wrong measured times. We
now report the accumulated times in microseconds on all platforms.
* Renamed a bunch of variables in main.cpp to be more descriptive.
* Removed unnecessary SDLMain.h and SDLMain.m OSX objective-c class as well as the plist, strings, xib and icns files, which are not needed.
* included cstdio in imguiRenderGL since SDL2 doesn't do it for us.
* Updated premake5 script to support SDL2 and to set the debug directory, as well as copy the SDL2.dll to the target directory on Windows.
* Updated readme with more descriptive, platform-specific demo project setup instructions
* Updated appveyor build script to build vs2015 as well.
* Updated Travis build script to build SDL2 from source, because they use Ubuntu 12.04 which doesn't have the libsdl2-dev package in its repositories.
