Using Javascript in GCS for UDFs

(Issue #642)

(Revising based on discussion).

For now in UDF magics we will still require the UDF to be defined in the cell (although it could be a thin wrapper). Importing code from GCS will be done through an extension in the comment block header (@import or similar).

We will not currently support code only in GCS, as we need to infer the schema and that may be intractable. Later we can add a way to specify metadata in GCS code to support this.

Expansion of UDFs should be moved to gcp.bigquery.Query, not gcp.data.SqlStatement, so that the latter has no knowledge of them. UDFs must be passed in with the udfs named argument.