Allow use of custom HttpClient

[tzifudzi]

As raised by @bmukokera in the Paynow developer forum, an SDK should follow SOLID principles and hence be easy to extend. Currently, the SDK uses an open source HttpClient from the Apache library and this cannot be changed without recompiling the original source code.

The SDK needs to be refactored to allow for use with a custom HttpClient. @bmukokera If you are still willing to take it up may I proceed to assign you the task? We currently do not have a set of contributing guidelines so I would suggest you just create a develop branch and make a pull request into master.