Add DelimFile new_reader and new_writer methods

[jdidion]

Implements #13 - new_reader returns a DelimFileReader instance and new_writer returns a DelimFileWriter instance.

DelimFileReader has a read method for reading one record at at time. It also implements Iterator, and read is really just an alias for next.

DelimFileWriter has a write method for writing one record at a time, and a write_all method for writing all records from an iterator.

DelimFile's convenience functions are re-written in terms of the new structs.

[nh13]

Thank-you, this looks great!