Adding row selection for keys in method get_all_records.

Adding a parameter `head` to the method `models.Worksheet.get_all_records` to be able to select the row where the keys are.
i.e: if a document have the first 2 rows with information that we don't want or we can't use as keys we specify the `head=3` and the 3rd row will be used as keys for each dictionary.