Start adding unit tests

[abdullahselek]

• With this commit there is a new folder named tests which is created to contain test classes. For now there is only one, which is for PKIBuilder. Coverage for PKIBuilder is now 100%. It's the first shape of the fix for #33.
• To install the dependencies for unit tests just run pip install -r requirements.testing.txt
• For running the tests you can use py.test -s -v command.