Added memoization to replace_formulae_with_calculated_values
Prepared for release
C version of excel_if should not return an error if the unused argument is an error
Fixed an error in the way that the ruby shim for the C version handled functions that return ranges (appears when trying to access named reference ranges)
Updated spec to correspond with changes in recent commits
Now transfer named references into cells to keep early in the process so that referenced but empty cells are kept in final results