Remove Werkzeug dependency

[dahlia]

Currently sqlalchemy-imageattach uses Werkzeug for two reasons:

• Context locals: we can simply bring it from Werkzeug as like Celery have done.
• WSGI app that serves static files: it’s only a small part of Werkzeug, so we can independently implement it again from scratch.

Werkzeug is a foundation of web frameworks, and there are similar ones for the same purpose e.g. WebOb. Users of web frameworks based on other foundations (for example, Pyramid is based on WebOb instead of Werkzeug) might get inconvenient by Werkzeug dependency.