implement Row

the first version
add method Row:fragment_length
Row's tests: a function to test any row sequence
add rows examples to test model.row
add method Row:text
Row() throws if first argument is empty
fix error in test row_spec
fix error in row_spec

tests were not applied :(
Row:text returns only one value

See string.gsub
add methods Row:block-2-[left|right|nearest]