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

DOC: minor edits in 0.11.1

DOC: cookbook/io.rst doc updates

TST: py3 issue on catching test warnings in test_parser_deprecated
Merge pull request #3693 from jreback/io_api

CLN: added io.api for i/o importing functions