Unify website slugify method behavior with or without python-slugify 

When you create a new page a related slug is created.
The website slugify method takes some help from a dedicated python module named 'python-slugify'
Python-slugify is not for now a mandatory odoo dependency, so there is a case when it's not here to build a slug.
The problem is that this slug is not generated the same way depending on 'python-slugify' presence. These patch purpose a solution to address this. 

    Impacted versions:

        7.0 and above

    Steps to reproduce:

        BE SURE TO NOT HAVE 'python-slugify' in your python project path 
        Create a page with a title containing caps and special chars, for example "Do YOU know 'Martine à la plage' ?"
        Click continue

    Current behavior:

        The generated slug looks like "Do-YOU-know-Martine--la-plage-"
        That's quite bad for an url slug pattern...

    Expected behavior:
        The result should be the same as with python-slugify
        "do-you-know-martine-a-la-plage"

I've also add some unit tests in order to ensure that behavior are exactly the same with or without 'python-slugify'
Hope in a nearly future 'python-slugify' will be marked as mandatory dependency. It does the job quite well, so it's not necessary to mock it's behavior.
You need to import that also when slugify is present as we execute the fallback code when we catch a TypeError.