Added the EtcdResult.leaves property (returning a generator), and the
EtcdResult.get_subtree() method to fetch all objects in a recursive fetch.
Change children to leaves in tests; clarify docs

This closes #30 as well.

Signed-off-by: Giuseppe Lavagetto <lavagetto@gmail.com>
Merge pull request #37 from jplana/rename-and-extend-children

Rename and extend children