Adds support for go modules outside of GOPATH

[bheatwole]

Fixes #547

I have:

• [*] Added tests covering the bug / feature
• [*] Updated any relevant documentation
  The 'Getting Started' guide needed a few touch-ups to remove 'dep' and use 'go mod'

If you follow the steps in the Getting Started guide, or the Steps to Reproduce listed in #547, be aware that you will need to add the following command after running go mod init

go get github.com/99designs/gqlgen@[commit ID of this PR]

Otherwise go mod will pull in the latest release of gqlgen that doesn't have the fix! Once this PR is merged and released, that extra step should no longer be necessary.

Also I added a line to 'Getting Started' to mkdir server. If that directory did not exist, the generator would panic. I fixed it in the documentation because I wasn't sure if that was the desired behavior or if the template system should be creating the directory if it doesn't exist.

[vektah]

Nice clean pull request, however this is already supported on the next branch, you can test it out using go get github.com/99designs/gqlgen@next.

see #514 and #226