Support Google login (two-factor auth) (smogon#999)

This doesn't support setting accounts up for Google login: that still
has to be manually done via the database by setting the email field to
`[email protected]@`, where the second `@` denotes that it's using
Gmail login.

If the email field does end in `@`, `getassertion` will note this by
sending `;;@gmail`, to convey that the server is expecting a Google
login token rather than a password.

Upon receiving `;;@gmail`, the client will replace the password box will
with a Google login button, and then send the resulting Google login
token to the server in the `password` field. The server will validate
the "password" using the Google server libraries, and otherwise handle
the login as normal.

Note that Google login requires various features that a paranoid person
might disable; most notably 3rd-party cookies.

Fixes smogon/pokemon-showdown#3394

.gitignore

@@ -11,3 +11,4 @@ eslint-cache/
 .DS_Store
 Thumbs.db
 npm-debug.log
+/vendor/

composer.json

@@ -0,0 +1,5 @@
+{
+    "require": {
+        "google/apiclient": "^2.2"
+    }
+}