Configurable way to get children from a node

Now all functions in compare take an extra `get_children` argument. This
should be a function which controls how children are extracted from tree
nodes. By default it is `lambda node: node.children` for backwards
compatibility.

Signed-off-by: Tim Henderson <tim.tadh@gmail.com>
Removed dead code: post_traverse

This method is no longer used. Therefore, I removed it and its test. I
am putting this commit in the branch for the API change as the change to
its API identified it as dead code.

Signed-off-by: Tim Henderson <tim.tadh@gmail.com>
Added get_label, and label_dist paramaters.

These allow the distance to be more generic. See the docs on
`zss.distance`.

Signed-off-by: Tim Henderson <tim.tadh@gmail.com>
Documentation improvements for new API.

Signed-off-by: Tim Henderson <tim.tadh@gmail.com>
Update my email address
Merge pull request #14 from timtadh/feature/API_get_children_change

Feature/api get children change