bigquery: some breaking changes

[Jonathan Amsterdam]

Three changes to the BigQuery client today. You can vendor to tag v0.5.0 to isolate yourself.

The largest change is the removal of the ValueList type. It is no longer necessary.

Instead of 

var v ValueList
... it.Next(&v) ..

use

var v []Value
... it.Next(&v) ...

The other two changes apply to schema inference (the bigquery.InferSchema function):

• A field of type []byte will result in the SQL type BYTES. Previously it was STRING.
  
• The types uint, uint64 and uintptr are no longer supported in schema inference. BigQuery's integer type is INT64, and those types may hold values that are not correctly represented in a 64-bit signed integer.