Use paths relative to current working directory

Previously, using the construct `__DIR__ . '/../data/' for paths to keys
causes fatal errors immediately when the user does not override the
configuration, and instead uses the default configuration. This is
particularly problematic because the documentation suggests that running
`vendor/bin/generate-oauth2-keys` should be sufficient.

This patch modifies those paths to be relative to the current working
directory, `getcwd() . '/data'`, fixing those issues.

Author: weierophinney

config/oauth2.php

@@ -10,9 +10,9 @@
  * @see http://php.net/manual/en/class.dateinterval.php
  */
 return [
-    'private_key'    => __DIR__ . '/../data/private.key',
-    'public_key'     => __DIR__ . '/../data/public.key',
-    'encryption_key' => require __DIR__ . '/../data/encryption.key',
+    'private_key'    => getcwd() . '/data/private.key',
+    'public_key'     => getcwd() . '/data/public.key',
+    'encryption_key' => require getcwd() . '/data/encryption.key',
     'access_token_expire'  => 'P1D', // 1 day in DateInterval format
     'refresh_token_expire' => 'P1M', // 1 month in DateInterval format
     'auth_code_expire'     => 'PT10M', // 10 minutes in DateInterval format