gunslinger v0.03-alpha

• Includes for gunslinger header-only library
• Project template for quick start (Win/Mac/Linux)

Instructions for template:

* Gunslinger library is included in `third_party/include`

# win (msvc):
    * open `x64 Native Tools Command Prompt for {2017/2015}`
    * `cd` to root directory for template
    * run `proc\win\compile_win_cl.bat`
    * executable will build and be placed in `bin`
    * run `bin\App.exe`

# win (mingw):
    * open `git bash`
    * `cd` to root directory for template
    * run `bash ./proc/win/compile_win_mingw.sh`
    * executable will build and be placed in `bin`
    * run `./bin/App.exe`

# linux (gcc):
    * `cd` to root directory for template
    * run `bash ./proc/linux/compile_linux_gcc.sh`
    * executable will build and be placed in `bin`
    * run `./bin/App.exe`

# osx(gcc):
    * `cd` to root directory for template
    * run `bash ./proc/osx/compile_osx_gcc.sh`
    * executable will build and be placed in `bin`
    * run `./bin/App.exe`