Adopt FlaskScopeManager

[rmfitzpatrick]

These changes introduce an optional scope manager using Flask's stack, as originally proposed in https://github.com/opentracing/opentracing-python/pull/105.

This is helpful for ensuring that the active span obtained anywhere in a view function's call chain is guaranteed to be associated with request as provided by werkzeug's context locals.

[analytically]

@carlosalberto