ENH: add integer sheetname support in read_excel
closes #4301

I guess a sheet names as '0' would have to be specified as as actual string to avoid confusion
 yep. i should also add something to the docs acknowledging that and an example
 i'll add a test for that to see how it's handled. not sure how `xlrd` deals with that
 you could try to get a stringified first then fall back to positional if it is an integer
not sure if that is more confusing
 so essentially prefer named sheets to indexing where possible
 on 2nd thought I think passing an int try for index otherwise try named (like u have it)
no ambiguity
 @jreback ot what is the Bounty label for?
 Bounty label?
 there's an issue label called "Bounty", just wondering what that's for...
 no idea
maybe a standard label that git creates?
like bounty on stackoverflow
 or it could be the quicker picker upper :)
 :laughing:
 @cpcloud @jreback it's the label for issues with bounties on Bountysource ;) (https://www.bountysource.com/trackers/50755-pydata-pandas)
 @ccvergara oh cool didn't know about that thanks!
 @jreback any last words?
 I think u need to rebase as release notes 
looks older
 @jreback good 2 go?
