Allow accessing tree via nested urls. Fixes #134.

Now you can do:

`/instance/plot/{plot_id}`
and
`/instance/plot/{plot_id}/trees/{tree_id}`

If `tree_id` isn't a tree on `plot_id`, it will return 404.
Merge pull request #293 from ahinz/fix/134/nested-tree-urls

Allow accessing tree via nested urls. Fixes #134.