Index updates

* Define `show`, `showall` for `Index`
* Add versions of `replace_names` which allow replacement using `Dict`s, functions (ala Pandas)
* Bugfix: `replace_names!` needs to return the modified `Index` for `replace_names` to work
* Added `pop!`, `shift!`, `unshift!`, `append!`, `prepend!`
* Bugfix: `delete!` should return the deleted item (for parity with mainline julia, use by `pop!`, `unshift!`)
* Bugfix: `push!` should return the modified `Index` (for parity with mainline julia)
* Added tests for the above