Update `oauth_client_class`. Fixes "400 Bad Request" HTTP header issue.

This commit updates Manuel Lemos's `oauth_client_class` library to the
latest version, which fixes its handling of OAuth 1.0a such that the
Authorization HTTP header no longer duplicates HTTP request body
variables for POST requests. When the POST body is long, this caused an
overly long Authorization header to be sent, which caused Tumblr's nginx
server to respond with an unhelpful 400 Bad Request.

See especially the following threads:

* https://wordpress.org/support/topic/error-400-403
* http://www.phpclasses.org/discuss/package/7700/thread/159/

This commit also updates Tumblr Crosspostr's use of library to reference
its new external configuration file correctly.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@891557 b8457f37-d9ea-0310-8a92-e5e31aec5664
Check response from Tumblr before using in loop.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@891558 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme, version bump to 0.7.11.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@891559 b8457f37-d9ea-0310-8a92-e5e31aec5664