Replace PEAR's HTTP_OAuth with Manuel Lemos's library.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@870437 b8457f37-d9ea-0310-8a92-e5e31aec5664
Genericize base classes further.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@870438 b8457f37-d9ea-0310-8a92-e5e31aec5664