v0.3.0

Adds pdf(), the seventh method on Invoice and the only one returning raw bytes.

The authorizer's own rendering of an authorized document. The XML remains the legally valid document; this is a convenience, and the authorizer's endpoint for it is unstable by their own documentation — an APIError with status 502 means the authorizer is unavailable, not that the invoice is wrong. NFS-e only; NF-e answers 501.

Internal: the HTTP layer was split so the raw response is available, because the previous path parsed JSON off every response and would have corrupted a PDF. Behaviour-neutral for the six existing methods.