* 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.
Build scripts download SDL, download (or build) premake5, generate projects, then build, in several configurations and platforms
Project now builds with premake5 instead of premake4 to allow VS2015 project generation
Tweaked SDL directory dependency to work better with CI
Update README
