Windows; native extensions support

[FloorD]

Consolidates issues #91 #90 #84 #74 #59 #46

Traveling Ruby currently supports Ruby 2.1.5 and Ruby 2.2.0 for Linux and OS X. For Windows, only Ruby 2.1.5 is supported. This is because the RubyInstaller project hadn't released Ruby >2.1.5 binaries last time Traveling Ruby saw some work. We're a bit past that 2.1.x point now: https://rubyinstaller.org/

What is the necessary work for getting Native extensions working with Windows? #91
Document building for windows #90
Win32-api giving issue creating packages for Windows #84
windows version supported? #74
Another approach to Windows packaging #59 > which @FooBarWidget agreed might be a good approach
Native extensions support for Windows #46

[hanetzer]

@FloorD I'm unsure why RubyInstaller hasn't updated; I can build a 2.5 build on
gentoo for mingw-w64 using crossdev.

[starkgate]

There's also a 2.5 build on their website now.