Add merged cells functionality

[timon]

Allow user to merge cells on worksheet:

@sheet.merge_cells start_row, start_column, end_row, end_column

[zdavatz]

Timon: Can you please write a test for this.

Best
Zeno

[johanlunds]

I made some further changes to timon's fork. Also untested. But still, might be of interest to anyone that has the same problem I had. I implemented support for saving merged cell-edits when editing an existing file. Technically, #write_merged_cells will now also be called in Writer::Worksheet#write_changes. It was only called in #write_from_scratch before. My commit.