add row support (including delete)

A very basic row support. Now you can:

```python
import gspread

# Login with your Google account
gc = gspread.login('thedude@abid.es', 'password')

# Open a worksheet from spreadsheet with one shot
wks = gc.open("Where is the money Lebowski?").sheet1

for row in wks.list_rows():
  # get row value for a given column header, ie. first_name:
  print row.first_name

  # delete a row if it matches your conditions:
  if row.first_name == 'John':
     wks.delete_row(row)
```

This should fix #13