Added lume.invert()
Added tests for lume.invert()
Changed lume.set() to make use of lume.invert()
Added empty-table test for lume.set()
Moved lume.invert() to be near lume.clone()

This makes the series of python-like functions (map, all, any, reduce,
set, filter) consecutive in the source and readme, which seems like a
more logical order.