[#217] Cleanup unused code
[#217] Use HttpURLConnection

* Replace HttpClient appearances
[#217] Broaden custom exception usage
[#217] Reuse HttpUtil for generic requests
[#217] Reuse HttpUtil components

* Use public interfaces for reusable functions
[#217] Fix POST method

* call getInputStream() before getStatusCode()
Merge pull request #218 from akvo/issue/217-refactor-http

Code reviewed