Provide Access to Response Headers

[naaman]

There's no mechanism for accessing the response headers. This issue is primarily driven by the need to check rate limit headers sent from the API. However, there could be other headers that need to be exposed.

A couple discussed options:

App app = connection.execute(new AppCreate(), apiKey);
app.getResponse().getHeaders();

or

App app = connection.execute(new AppCreate(), apiKey);
List<Header> headers = connection.getLastHeaders();

and for ease of use:

App app = herokuApi.createApp();
RateLimit limit = herokuApi.getRateLimit();