ENH : add ability to change image size

 - added function init_artists() which takes a new base
   image and updates all the artists/limits/auxilary data
   to deal with images of the new size

 - added functions to connect/disconnect (connect_callbacks,
   disconnect_callbacks) all of the call backs for the cursor/parasite
   axes updates.  This is needed for use with enmal where the figure is
   constructed without a canvas (it is added eventually).
DOC : added docstring to init_artists + comments

 - added docstring
 - fixed up/removed misleading in-line comments
Merge pull request #2 from tacaswell/refac_xs

Refac xs