Handle out-of-range condition where there are more data cells in a row
than there are headers.
Add a lot of documentation.

Fix IndexError for HXLRow.getAll()

Add HXLDataset.__str__()
Start implementing tests for default schema.
Added support for default schema (not yet in command-line script).
Include schema data file.
Switch to setuptools from disttools.

Install default core HXL schema for use with hxlvalidate script.