ENH: allow layout for grouped DataFrame histograms

Also remove the slow decorator for tests that take < 100 ms time
ENH: add integer sheetname support
Merge pull request #4104 from cpcloud/layout-with-by

ENH: allow layout for grouped DataFrame histograms
BUG: period idx str map correctly returns an array of str with object dtype
Merge pull request #4136 from cpcloud/period-index-repr

BUG: period idx str map incorrectly returns a str repr of itself when ind.map(str) is called
Merge pull request #4308 from cpcloud/read-excel-page-number-sheet-name

ENH: add integer sheetname support in read_excel