DOC: revamped dtypes section in basics.rst
     fixed removal of foo temp files in 10min

DOC: added to time series in 10min.rst
DOC: added sorting examples to 10min
BUG: fixed multi-index selection via loc, back to using some
     of ix code (but still do validation if not mi)

ENH: add xs to Series for compatiblity, create _xs functions in all objects