Reimplement URLPattern as str subclass

[Suor]

This has some advantages:

• it's now usable outside urljects helpers such as Django url() or really anywhere
• code is cleaner
• fixes mutability bug

On last one: now you modify parts, which can lead to unexpected behavior:

base_url = U / 'base'
u1 = base_url / '1'
u2 = base_url / '2'  # now it's /base/1/2

[Visgean]

I will make a new release in couple of days.

[Visgean]

Relased. :)