Experimental patch for evaluating the string with Python's str.format based on discussion with @venomgfx,
not really pushing this to go into master - just including for reference.

Example stamp note:

tiles: {scene.render.tile_x}x{scene.render.tile_y}, shift: {camera.data.shift_x}x{camera.data.shift_y}, ver: {bpy.app.version_string}

... evaluates to: tiles: 64x64, shift: 0.0x0.0, ver: 2.77 (sub 0)

This is an equivalent of doing:

scene.render.stamp_note_text.format(**dict(scene=context.scene, camera=scene.camera, bpy=bpy, context=context))

Possible Improvements:

• An error in one expression could be made so only the text between the {} fail.
• error handling could be better in general. currently errors just show in console.