ENH: add .iloc attribute to provide location-based indexing

Updated to include new indexers:

``.iloc`` for pure integer based indexing
``.loc`` for pure label based indexing
``.iat`` for fast scalar access by integer location
``.at`` for fast scalar access by label location

Much updated docs, test suite, and example

In the new ``test_indexing.py``, you can change the ``_verbose`` flag to True to get more test output
anybody interested can investigate a couple of cases marked ``no comp`` which are where the new
indexing behavior differs from ``.ix`` (or ``.ix`` doesn't work); this doesn't include cases where a ``KeyError/IndexError`` is raised (but ``.ix`` let's these thru)

Also, I wrote ``.iloc`` on top of ``.ix`` but most methods are overriden, it is possible that this let's something thru that should not, so pls take a look

*Please try this out and let me know if any of the docs or interface semantics are off*