Merge pull request #2947 from jreback/dtypes_doc

DOC: updated dtypes docs
ENH: add .loc attribute to provide location-based indexing

TST: added multi-index tests

DOC: changed loc -> iloc
     added more docs

ENH: added integer lists as indexers to iloc

ENH: raise correctly on out-of-bounds slicing
     support negative indexing in iloc and icol
CLN: move all indexings (ix/iloc) to PandasObject in generic.py
     (except _SeriesIndexer in series.py)
     add name parameter to Indexer creation, makes indexers independent
     of their external names
TST: new test suite for indexing
ENH: added loc/at/iat indexers ....almost done
DOC/TST: revised indexnig section in docs
         updated whatsnew
         all tests work