Move the value paremeter of the Value init method

The value parameter of Values init method is now moved to the end of the parameter list. This means that Value(5) is now equivalent to Value(data=5).

This fixes #18
Launch test suite with setuptools.

call `python setup.py test` in order to run all unit tests.

This fixes #10
Merge pull request #25 from stoewer/master

Move the value paremeter of the Value init