bigquery: BigQuery.writer return type should be public.

[tswast]

The BigQuery.writer function does not link to its return type in the
javadocs.

http://googlecloudplatform.github.io/google-cloud-java/0.5.1/apidocs/com/google/cloud/bigquery/BigQuery.html#writer(com.google.cloud.bigquery.WriteChannelConfiguration)

I believe this is because TableDataWriteChannel is package-private.
Since the snippets use BaseWriteChannel<BigQueryOptions, WriteChannelConfiguration> as the variable type, I changed the type to
this public type rather than making TableDataWriteChannel public.

[tswast]

@mziccard @lesv Maybe this method should return http://googlecloudplatform.github.io/google-cloud-java/0.5.1/apidocs/com/google/cloud/WriteChannel.html instead? In that case, I'd want to update the snippets, too.

[lesv]

You probably should deprecate the existing version, then provide the new version. That's more or less the policy they've established.

[tswast]

Makes sense, though this isn't really a breaking change. No one could have used the return type it had before.

[lesv]

If writer is fairly recent, then I'm ok with out the @Deprecated, but if it's old, then we should do it, despite it being unused.

[coveralls]

Coverage increased (+0.005%) to 84.255% when pulling 58d6d2b on tswast:tswast-bigquery into 8ec721e on GoogleCloudPlatform:master.

[tswast]

Closing based on #1392 (comment)

We'll probably make TableDataWriteChannel public instead.