Let the field spec be either a string or a non-string sequence (suggested by Martin Blais with use cases).
Improve the error message in the case of a SyntaxError (caused by a duplicate field name).

Add maxlen support to deque() and fixup docs.
Partially fix __reduce__().  The None as a third arg was no longer supported.
Still needs work on __reduce__() to handle recursive inputs.

Allows optional commas in the field-name spec (help when named tuples are used in conjuction with sql queries).
Adds the __fields__ attribute for introspection and to support conversion to dictionary form.
Adds a  __replace__() method similar to str.replace() but using a named field as a target.
Clean-up spelling and presentation in doc-strings.