Add support for unvalidated text fields

[pbchase]

This PR is a bit big. It addresses issues #3 and #7. It also forced a redesign in get_one_rectangle_of_values() which forced some changes in get_long_categorical_field_response_values(). Sorry for the size, but it's an important PR. It will allow faster development of the text validation types.

Here are the details of the commit:

• Address issues Add a function for filling text fields #3, Add a function get_long_text_fields.R #7.
• Add get_long_text_fields().
• Add get_long_text_field_values().
• Modify get_long_categorical_field_response_values().
• Modify get_one_rectangle_of_values()
• Update proof_of_concept().
• Update README to reflect the new feature.