Fix pingback plugin

Commit b33fc0c88b1 removed xmlrpc.py which is needed for the pingback
plugin. Add it again and update it to the latest API.
The pingback plugin itself also needed several small fixes.
Add gravatar support to the comments plugin

Adds a $cmt_gravatar template variable to the comments section which
provides a gravatar from gravatar.com.
Fix pingback spam checking

The call to the akismet spam checking did not include the remote
IP-address. This is a required field for Akismet so add it.
Fix gravatar call

The last commit was from an old stash. Remove the icon size to always
get the default site (currently 80x80) back from gravatar.
More xmlrpc cleanups

Remove the remaining sys.stdout.write calls and use the response.write
method instead.
Fix last commit

While changing around the variable names in the last commit,
we actually broke the code in the except case.
Merge 'upstream/master' into fix-pingback

Conflicts:
	Pyblosxom/plugins/xmlrpc_pingback.py
Merge branch 'fix-pingback' into personal
Merge branch 'comment-add-gravatar' into personal

Conflicts:
	Pyblosxom/plugins/comments.py