Began segmented_sieve code

The heart of the algorithm has been implemented.  Now I need to make some
structural changes to the code.  It would be useful to change the function
to a generator and to improve the modularity of the whole praxis package.
Finished 108, changed directory names, added init

I finished exercise 108.

I added 'ex' to the directory names so that they can be imported.

I added __init__ files to all the directories so that the whole package can
be used as a module.