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_
