Retrieve name of medium image thumbnail via xmlrpc

As the first step in supporting the use of automatically generated
Wordpress thumbnails (org2blog issue #158), use WordPress XMLRPC
to retrieve media information, including names of automatically created
thumbnail images.
Read from three-part image cache string
Replace <img> tag with href and img thumbnail
Image thumbnail behaviour now optional

Image thumbnail behaviour can now be configured using
org2blog/wp-image-thumbnails, which is by default off.