Add support for godep to pin project dependencies

[godep](https://github.com/tools/godep) appears to be solid choice for pinning Go project dependencies. This pull request contains a `Godeps` file with SHAs of dependency revisions. Tests were run at the specified SHAs before freezing.

To restore the state frozen in `Godeps`, first install `godep`:

```bash
$ go get github.com/tools/godep
```

Then, run:

```bash
$ godep restore
```

**Note**: This is going to change the `HEAD` of dependencies in `$GOPATH/src`.

As you make changes to the code, use the following command to invoke the test suite:

```bash
$ godep go test
```

To [add](https://github.com/tools/godep#add-a-dependency) or [update](https://github.com/tools/godep#update-a-dependency) dependencies, please see the `godep` documentation.

Aims to resolve: https://github.com/OpenTreeMap/ecobenefits/issues/29