Subtree merge

[1l0]

This adds following features:

• Users don't have to clone godot_headers by their hands any more.
  • Just go get github.com/ShadowApex/godot-go/godot
• go get means go install so the build time for dynamic libraries would be drastically improved other than that users manually go install on the godot/ directory in advance.

How to update godot_headers/:

git pull -s subtree godot-headers master

[ShadowApex]

Hi @1l0 ,

This looks great. I should be able to merge this if we can fix the automatic build process. Right now this change would break Travis CI's builds: https://travis-ci.org/ShadowApex/godot-go/jobs/318828482

Right now it fails with this:

$ git clone https://github.com/GodotNativeTools/godot_headers.git
fatal: destination path 'godot_headers' already exists and is not an empty directory.

Just need to update .travis.yml to remove the steps for linking and cloning of the godot headers.

[1l0]

It might be fixed.