Better File support for pyramid: Range, Conditional request and Etag

[inkhey]

We do need to extend file support for pyramid (and later for other framework), in order
to support range query.
As pyramid does support a "conditional" mecanism, we will use it to handle all supported feature:

• Range
• If-Modified-Since
• If-None-Match + Etag

see https://docs.pylonsproject.org/projects/webob/en/stable/api/response.html#webob.response.Response.conditional_response_app

This is the first step to have better file support in hapic, we should find a way to implement those feature later for other framework (see #171).