Added support for setting headers and overriding or extending the default Faraday connection block before a connection is constructed.

From the updated README:

By default, Hyperclient adds `application/json` as `Content-Type` and `Accept` headers. It will also send requests as JSON and parse JSON responses. Specify additional headers or authentication if necessary.

```ruby
api = Hyperclient.new('https://grape-with-roar.herokuapp.com/api') do |client|
  client.headers['Access-Token'] = 'token' # this is new, you can access headers before the connection is built
end
```

By default, Hyperclient constructs a connection using typical [Faraday](http://github.com/lostisland/faraday) middleware for handling JSON requests and responses. You can specify additional Faraday middleware if necessary.

```ruby
api = Hyperclient.new('https://grape-with-roar.herokuapp.com/api') do |client|
  client.connection do |conn|
    conn.use Faraday::Request::OAuth # this is new, you can add middleware to the default Faraday connection block
  end
end
```

You can build a new Faraday connection block without inheriting default middleware by specifying `default: false` in the `connection` block.

```ruby
api = Hyperclient.new('https://grape-with-roar.herokuapp.com/api') do |client|
  # this is new, you can just say default: false and not inherit anything, this also lets us specify other options in the future
  client.connection(default: false) do |conn|
    conn.request :json
    conn.response :json, content_type: /\bjson$/
    conn.adapter :net_http
  end
end
```

You can modify headers or specify authentication after a connection has been created. Hyperclient supports Basic, Token or Digest auth as well as many other [Faraday](http://github.com/lostisland/faraday) extensions.

```ruby
api = Hyperclient.new('https://grape-with-roar.herokuapp.com/api')
api.digest_auth('username', 'password')
api.headers.update('Accept-Encoding' => 'deflate, gzip')
```

You can access the Faraday connection directly after it has been created and add middleware to itpoint. As an example, you could use the [faraday-http-cache-middleware](https://github.com/plataformatec/faraday-http-cache).

```ruby
api = Hyperclient.new('https://grape-with-roar.herokuapp.com/api')
api.connection.use :http_cache
```