Fix "Object should be of type List" errors when adding valid context
menu items

This is stupid and should not be necessary, or even work but it does.
Apparently XBMC has some weird problem with single element collections.
It causes perfectly valid lists to (sometimes) fail when passing them to
add_item. Don't ask me why using context_menu_items in a log statement
changes anything, but it does. I can only imagine that internally XBMC
recognizes that it's a single item list and corrects itself.
Fix unicode problems in build_plugin_url

urllib.urlencode() didn't handle unicode characters in parameters. Fixed
by looping through params and converting unicode keys to utf-8 Also
forced an error if the string is any encoding but utf-8
Merge pull request #3 from bstrdsmkr/master

Fix unicode problems in build_plugin_url