Added Coinbase custom connection support for thrid party system that cannot use default URL connection implementation

Hi,

I tried to use official library on Google AppEngine and I found out that URLConnection urlConnection = url.openConnection() does not return HttpsURLConnection, this caused that library could not be used.

So I prepared fix for your official, so that Google AppEngine users can inject their CoinbaseConnection implementation which uses Google URL Fetch Service and wrapped your offical way in CoinbaseConnection implementation CoinbaseConnectionImpl.java. I added CoinbaseBuilder().withCoinbaseConnection(connection) function to inject custom implementation. Your tests are still OK.

Could you pull changes to official library? If you will, I will prepare and release package coinbase-java-appengine to add support for Google App Engine users.

Thanks,
Dean Gostiša