* Generate protos for Models API.

The Models API is a component of the bigquery_v2 interface.
It is not available as a gRPC API, but it does provide
protocol buffers. This commit adds those protocol buffers
to the client so that they can be used to avoid much manual
work to create resource classes that can be serialized to/from
JSON.

* Fixes BQ Models API generated code for Python 2.7

* Format quoted strings as plain text.

* Add handwritten model API classes.

These classes provide the top-level classes for the Model API. The
protocol buffer objects are used for all sub-objects.

The pattern for mutable properties follows the same as with Table and
Dataset: a `_properties` dictionary contains the property values in the
REST API format.

* Address review comments in Model.

Use setdefault for labels.
Simplify setter logic for expires.

* Combine error messages for too many and too few parts in model/table ID.

* Regen BigQuery Model API classes.

This commit is auto-generated by:

    SYNTHTOOL_GOOGLEAPIS=$HOME/src/googleapis-private python -m synthtool

* Update test for Model to new representation of TrainingRun.startTime.

* Add client methods for Models API.

* Adds hack to workaround milliseconds format for
  model.trainingRun.startTime.
* Adds code samples for Models API, which double as system tests.

Even if the Models API itself is considered GA, I'd like to mark the
client methods as Beta for a time to give us some more ability to make
breaking changes until we're sure of the public interface we want to
expose.