Feature/adding method period substract

[Bibendus83]

This method allows to substract 2 Period objects like the difference operation in sets.
Compared to Period::diff, this method operation is not commutative and returns only one possible result.

The method returns a Sequence that can contain from 0 to 2 Period objects.

This method will be required in a future Sequence::substract method
For major details check ticket #78

[Bibendus83]

If you confirm this PR I can go on with the Sequence::substract method that requires Period::substract.
I hoped to implement this function in a better way but I saw that the diff function is a bit more complicated than I tought so I decided to use it and filter the result I needed.

[nyamsprod]

• I've merge your PR
• I've slightly updated your implementation to make it more readable IMHO
• I've moved the tests to the proper subdirectory
• I've used your illustration while adding the method to the package documentation