Unit testing

Hello Ahmed.
I've found your project useful.
I want to use your library for a little work project where I need to export data from some DBF files (written in different dbase format versions). Looking at the code, I think I can cleanup and refactor some parts, but before proceeding, I want to add some unit tests to make my refactor bug safe.
In this pull request you will find stub project for unit tests, and some initial tests to DbfColumn.
If you are interested in my contribution, please let me know.

Have a nice day! :)
Nando