Add `installdeps` command.

So you can do `./gp installdeps` rather than:

```
GOPATH="`pwd`/.gopack/vendor" go install \
  github.com/bmizerany/assert            \
  github.com/ParsePlatform/go.grace      \
  github.com/technoweenie/grohl          \
  ...
```

/cc @d2fn, @technoweenie