CLN: added io.api for i/o importing functions
closes most of #3411 (all except the deprecation of `from_csv`)
- 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
