deepKeys: treat empty object as a leaf node

prior to this, that test case was returning an empty array
style
deepKeys: rewrote recursively
deepKeys: added hella tests
deepKeys: convert unnecessary base case to comment
Merge pull request #6 from Clever/recursive-deepKeys

Structurally recursive deepKeys
Merge pull request #5 from Clever/empty-leaves

deepKeys: treat empty object as a leaf node