Code cleanup, better error handing, unittests and more examples

Nothing really changed since the last pull-request, except this one should merge without issues.

Updates:
- The code got some cleanup to be more consistent and provide an API more similar than that of existing extensions (ex. mysqli).
- The more common errors are handled for connect, selectPool and write.
- A additional file with UnitTests for the OO interface has been added.
- Some more examples are added to show the uses as also a Readme.md file providing an overview of the examples.