Add basic functional tests

This pull request implements a functional test client and ports the Python smoke tests to Go. It's not currently integrated with the server, so Simple Push must be started manually before running the tests. Ideally, we'd have a mechanism for creating a `simplepush.Application` instance programmatically; this should be easier once #30 is done.

The tests can be run via `GOPATH="$(pwd)/Godeps/_workspace:$(pwd):$GOPATH" go test -v github.com/mozilla-services/pushgo/client`.

Closes #32. @bbangert, @jrconlin r?
sleep is tricky, we can't presume the test will always take more or less this time. How is the simplepush program terminated? It's ok to exclude this from this PR so that we can setup 'go test' to spin it up appropriately for testing.