Add ExcelFile to API docs + clarify ExcelFile isn't deprecated (just moved)
ExcelFile the class isn't actually in the API anywhere. We should rectify that.  Also, I realized from [this SO question on ExcelFile](http://stackoverflow.com/questions/19760741/how-do-you-read-excel-sheets-using-index-instead-of-name-with-pandas-read-excel/19778206) that the release notes aren't explicitly clear that `ExcelFile` is just _moved_ not deprecated.
