This commit re-adds the `testdata` folder that godeps strips and
also updates the `Makefile` to add a `vendor` target that will
handle this in the future.
This commit moves dependency management to use godep and the
new experimental vendoring support in Go 1.5.
NOTE: You must set `GO15VENDOREXPERIMENT` in your bash environment
for vendoring to work properly! See the updated `README` for details.