add GPM to track dependencies and easy installation

This is useful in two scenarios:

1. Deployment. You want to automatically compile the ent binary and your
dependencies changed. Instead of duplicating all dependencies necessary
for installation you can run `gpm` from your provisioning tool of choice
and be done with it

2. Setup for new developers. Instead of having to dig through the code
all you have to do is to run `gpm` and all dependencies are installed
globally.

GPM also has the nice side effect of installation stuff globally instead
of vendoring them, so `go test ./...` while developing doesn't spam your
CLI with the tests of your dependencies.
Merge branch 'feature/gpm'