Add ability for Logging (Issue #27) (2)

[yonojoy]

This implements the ability for logging, as described in issue #27 and discussed in pull request #29.

I recommited a pull request because I cleaned up my repository (I'm slowly starting to better understand git...) and can now provide a cleaner request that only contains one commit.

As discussed in pull request #29 this request contains code in OpenApiHttp for Delphi versions greater than XE4, that I cannot compile here (because Delphi XE4 does not provide System.Net.HttpClient and System.Net.URLClient and I am still using Delphi XE4). So please handle with care.

[Irame]

Hi, I would love to have the ability to write logs as well.

There is a minor issue with your implementation that causes it to not compile.
The call to LogRequest here does not work because SourceStream is a TStream and not a TStringStream. I would suggest to change the type of SourceStream to TStringStream to keep the encoding information.

Another feature I am missing is the ability to log the headers of the request.

Aside from that, I really like it and would love to see it merged!