Add ability to use first sheet in workbook if no name specified

[crisberrios]

One scenario is when processing many spreadsheets and sheet name is generated, so specifying it in a string reduces flexibility.

[ajepst]

It's easy enough for the user to pull the first sheet name and use it, and the ability to get the sheets is already part of the library (so it's not really less flexible this way, as in a user can already implement your scenario pretty easily just with 2 or so more lines), but sure, using the first sheet is a reasonable default. Seems like a good post-1.0 bonus feature.