Add extschema

test the various differences in backends for an extension like this.

Allow urlencoding on bytea inputs. In general urlencode(text) will end up outputting encodings based on the server encoding (usually utf-8, but no guarantees). This change allows the urlencoded output to have a deterministic encoding by passing the text through convert_to() first to get an encoded binary string. Closes #124

Allow setting CURLOPT_USERAGENT