Update pandas.read_gbq docs to point to pandas-gbq

The `pandas-gbq` package must be installed to use `pandas.read_gbq`.
Also, with soon-to-be-release version 0.3.0 of `pandas-gbq` the Google
Cloud client library is used instead of the Google API library.

Also, standard SQL is no longer beta. In fact it is highly recommended
over using legacy SQL.

Author: tswast

pandas/io/gbq.py

@@ -29,9 +29,11 @@ def read_gbq(query, project_id=None, index_col=None, col_order=None,
     The main method a user calls to execute a Query in Google BigQuery
     and read results into a pandas DataFrame.
 
-    Google BigQuery API Client Library v2 for Python is used.
-    Documentation is available `here
-    <https://developers.google.com/api-client-library/python/apis/bigquery/v2>`__
+    This function requires the `pandas-gbq package
+    <https://pandas-gbq.readthedocs.io>`__ which uses the `Google Cloud
+    BigQuery Client Library for Python
+    <https://googlecloudplatform.github.io/google-cloud-python/latest/bigquery/usage.html>`__
+    as of version 0.3.0 of the ``pandas-gbq`` library.
 
     Authentication to the Google BigQuery service is via OAuth 2.0.
 
@@ -70,7 +72,7 @@ def read_gbq(query, project_id=None, index_col=None, col_order=None,
 
     dialect : {'legacy', 'standard'}, default 'legacy'
         'legacy' : Use BigQuery's legacy SQL dialect.
-        'standard' : Use BigQuery's standard SQL (beta), which is
+        'standard' : Use BigQuery's standard SQL, which is
         compliant with the SQL 2011 standard. For more information
         see `BigQuery SQL Reference
         <https://cloud.google.com/bigquery/sql-reference/>`__