DOC: use na_rep not nanRep in .to_csv()

nanRep works but is deprecated and emits a FutureWarning.
Merge pull request #3683 from jreback/dup_insert

BUG: allow insertion/deletion of columns in non-unique column DataFrames
Merge pull request #3700 from tdsmith/patch-1

DOC: use na_rep not nanRep in .to_csv()
BUG: Deprecation error should not be fatal

DataFrame.to_csv() should succeed with a warning if nanRep is specified. Before this fix, to_csv threw a warning but did not save the file.
TST: test for to_csv to succeed with deprecated options (GH3701)
BLD: setup.py should state 0.11.1 not 0.12
BUG: restore 10.1 expand_repr behaviour, only for < max_cols, if wider then term GH3706
Merge pull request #3713 from y-p/PR_ode_to_joy

BUG: restore 10.1 expand_repr behaviour, only for < max_cols, if wider then term GH3706
CLN: added io.api for i/o importing functions

     moved excel functionaility out of io.parsers to io.excel
     added read_excel top-level function
       aliases from pandas.io.excel
     added read_stata top-level function, to_stata DataFrame method
       aliases from pandas.io.stata
       removed read_dta (replace by read_stata)
     added read_sql   top-level function, to_sql DataFrame method
       aliases from pandas.io.sql

DOC: doc updates for all the above and intro section to io.rst
TST: provide warning when using ExcelWriter/ExcelObject if from deprecated path