Allow Accept header to be set via file name extension

E.g., a URL like /some/resource.json will set the Accept header to
application/json. The request's PATH_INFO will be modified to remove the
extension before it reaches application code.

Enabled via the `tangled.app.set_accept_from_ext` setting (enabled by
default).
Represent dates as timestamps in JSON

This will be done only if both of the `json.encoder` and
`json.encoder.default` settings are not set.