BUG/ENH: Make names, levels and labels properties.

* `names` is now a property *and* is set up as an immutable tuple.
* `levels` are always (shallow) copied now and it is deprecated to set directly
* `labels` are set up as a property now, moving all the processing of
  labels out of `__new__` + shallow-copied.
* `levels` and `labels` are immutable.
* Add names tests, motivating example from #3742, reflect tuple-ish
  output from names, and level names check to reindex test.
* Add set_levels, set_labels, set_names and rename to index
* Deprecate setting labels and levels directly

Similar to other set_* methods...allows mutation if necessary but
otherwise returns same object.

Labels are now converted to `FrozenNDArray` and wrapped in a
`FrozenList`. Should mostly resolve #3714 because you have to work to
actually make assignments to an `Index`.

BUG: Give MultiIndex its own astype method

Fixes issue with set_value forgetting names.
ENH: Additional keyword arguments for Index.copy()

* Index derivatives can set `name` or `names` as well as
  `dtype` on copy. MultiIndex can set `levels`, `labels`, and `names`.
* Also, `__deepcopy__` just calls `copy(deep=True)`
* Now, BlockManager.copy() takes an additional argument `copy_axes` which
  copies axes as well. Defaults to False.
* `Series.copy()` takes an optional deep argument, which causes it to
  copy its index.
* `DataFrame.copy()` passes `copy_axes=True` when deepcopying.
* Add copy kwarg to MultiIndex `__new__`