Hierarchical sort for todo lists

Hello,

Your plugin is quite nice, but I think that it miss a good sorting function
(to sort by project, context etc.) so I have written one.
If you like my contribution, feel free to add it to your plugin.
There are also a few personnal modifications in my version (I'm not asking you
to incorporate these ones):

+ Mapping are done using LocalLeader as they are in an ftplugin which is local
  by definition
+ I have done a small change on the color to make it looks better with my
  theme (slate)
+ I have changed the url in the README.markdown file


 Best regards
 David Beniamine