Cesar/upgrading okhttp

[CesarValiente]

----------- Note -----------

This branch (pull request) is a continuation and was started from the still active pull request: https://github.com/spark/android-app/pull/12 I did that because I'm developing using AndroidStudio and gradle and actually is the "standard" way now, so these two pull requests should be reviewed asap (and merged) so other developers (mainly all) can help to improve the app using the "usual" Android development way.

------ Stuff done in this pull request (starting commit id: 31fdc50 ) ---------

Okhttp has been updated and with it the rest of the libraries Spark android-app is using. The libraries are retrieved from Mavencentral.

The way the app was using urlconnection instances now has been updated, since they are created using a factory now.