Create Gomfile to manage dependencies

This uses gom (https://github.com/mattn/gom) to manage dependencies.
The resulting _vendor dir matches the existing vendor dir.
Use gom when building and running tests.

This migrates a lot of the logic that's currently in various scripts
into a Makefile.
Remove vendor directory.

Dependencies are now being managed by gom, so this is no longer needed.
Merge pull request #74 from alphagov/gom

Manage dependencies with gom.