Release v1.6.0 (#324)

* Update version and changelog for v1.6.0
* Update docs
diff --git a/docs/epy/toc-everything.html b/docs/epy/toc-everything.html
index f47bd32..83fb7d3 100644
--- a/docs/epy/toc-everything.html
+++ b/docs/epy/toc-everything.html
@@ -70,6 +70,15 @@
      >googleapiclient.schema._SchemaToStruct</a><br />  </div>
   <h2 class="toc">All Functions</h2>
   <div class="private">
+    <a target="mainFrame" href="googleapiclient._auth-module.html#authorized_http"
+     >googleapiclient._auth.authorized_http</a><br />  </div>
+  <div class="private">
+    <a target="mainFrame" href="googleapiclient._auth-module.html#default_credentials"
+     >googleapiclient._auth.default_credentials</a><br />  </div>
+  <div class="private">
+    <a target="mainFrame" href="googleapiclient._auth-module.html#with_scopes"
+     >googleapiclient._auth.with_scopes</a><br />  </div>
+  <div class="private">
     <a target="mainFrame" href="googleapiclient.channel-module.html#_upper_header_keys"
      >googleapiclient.channel._upper_header_keys</a><br />  </div>
     <a target="mainFrame" href="googleapiclient.channel-module.html#new_webhook_channel"
@@ -138,6 +147,12 @@
     <a target="mainFrame" href="googleapiclient.model-module.html#makepatch"
      >googleapiclient.model.makepatch</a><br />    <a target="mainFrame" href="googleapiclient.sample_tools-module.html#init"
      >googleapiclient.sample_tools.init</a><br />  <h2 class="toc">All Variables</h2>
+  <div class="private">
+    <a target="mainFrame" href="googleapiclient._auth-module.html#HAS_GOOGLE_AUTH"
+     >googleapiclient._auth.HAS_GOOGLE_AUTH</a><br />  </div>
+  <div class="private">
+    <a target="mainFrame" href="googleapiclient._auth-module.html#HAS_OAUTH2CLIENT"
+     >googleapiclient._auth.HAS_OAUTH2CLIENT</a><br />  </div>
     <a target="mainFrame" href="googleapiclient.channel-module.html#CHANNEL_PARAMS"
      >googleapiclient.channel.CHANNEL_PARAMS</a><br />    <a target="mainFrame" href="googleapiclient.channel-module.html#EPOCH"
      >googleapiclient.channel.EPOCH</a><br />    <a target="mainFrame" href="googleapiclient.channel-module.html#X_GOOG_CHANNEL_ID"