Get the path for the deepest matching element

We usually want to grab the path for the deepest (leaf) matching object
in the tree; this is especially true in iOS 7 since Apple changed
the accessibility hierarchy and inserted intermediate views or
accessibility elements in between.

Let’s modify `-rawAccessibilityPathToElement:favoringSubviews:` to
search for the deepest matching object instead of return the first the
method encounters.
Merge pull request #116 from inkling/nanouk/search-for-leaf

Get the path for the deepest matching element