Add support for retrying failed requests with exponential backoff.

This feature can be enabled by setting the `retries` configuration for the
`Google_Task_Runner` class:

~~~php
$client = new Google_Client();
$client->setClassConfig('Google_Task_Runner', 'retries', 5);
~~~

Once `retries` is configured, the following error types will be retried by
default:

 * Errors with a `500` or a `503` HTTP status code.
 * Errors with a `rateLimitExceeded` or a `userRateLimitExceeded` reason type.
 * cURL errors:
   * `CURLE_COULDNT_RESOLVE_HOST`
   * `CURLE_COULDNT_CONNECT`
   * `CURLE_OPERATION_TIMEOUTED`
   * `CURLE_SSL_CONNECT_ERROR`
   * `CURLE_GOT_NOTHING`

These error types can be configured using the `retry_map` configurations for
the `Google_IO_Exception` and `Google_Service_Exception` classes. The retry map
is an associative array which maps errors types to the number of times each
error can be retried (-1 can be used to retry an error indefinitely). For
example:

~~~php
$client = new Google_Client();
$client->setClassConfig(
    'Google_Service_Exception',
    'retry_map',
    array(
        // Don't retry 500 errors
        '500' => 0,
        // Retry 503 errors once
        '503' => 1,
        // Retry rate limit errors until we hit `retries`
        'rateLimitExceeded' => -1,
        'userRateLimitExceeded' => -1
    )
);
~~~

For convenience, `Google_Config` provides three constants for configuring retry
limits: `Google_Config::TASK_RETRY_NEVER`, `Google_Config::TASK_RETRY_ONCE`, and
`Google_Config::TASK_RETRY_ALWAYS`.