Update function genfromtxt: add keyword `nrows`

Add keyword `nrows` to specify the number of rows to read.
1. We should not use keywords `nrows` and `skip_footer` in the same time. 
2. If there's not enough lines (less than nrows) to read, an exception would raise.
You are missing a closing parenthesis to match the opening one after `ValueError`, and that is triggering the Travis errors.