Better differentiation between the two  distance funcs
Simplify simple_distance usage instructions further
Update README.rst
Removed requirement that node is a normal object

Before I required that each node was a normal python object such that an
arbitrary attributed could be added. I was adding an `_id` attr to while
building the AnnotatedTree. This was unnecessary. I now store those much
more sensibly in a seperate list.

Signed-off-by: Tim Henderson <tim.tadh@gmail.com>