Use glide to handle go dependencies

[hinshun]

Fix #908

[junegunn]

Merged, thanks. Note that I updated Makefile again to set up its own GOPATH, since I really like to be able to clone fzf in any directory and build it without having to care about GOPATH.

I also moved the main package and Makefile to the root directory (it was not possible in the past due to the existence of Ruby executable named fzf), and I hope this new layout is more conventional for a Go project.

[hinshun]

Personally, I like to include the vendor directory under version control. Then you can be GOPATH independent and also protect against your dependencies being deleted from github. If you want to also protect against dirty vendor changes, then you can write a unit test that checks the git diff with a fresh glide install.

You can also move glide from a go get ... to an environment setup like apt-get install .... Instructions found here: https://github.com/Masterminds/glide#install