Merge cells feature

Example of use:

```python
from docx import Document

doc = Document()
table = doc.add_table(6,6)
header_row = table.rows[0]
header_row.merge_cells()

# args can also be passed to merge_cells(mergeStart, mergeStop) in order to implement any kind of merge
# on the same row, such as:
table.rows[1].merge_cells(0,3)  # This will merge the cells indexed from 0 to 2.
```