Merge pull request #6 from jcushman/master

Handle ArchivalUrl subclasses.
refactor: cleanup the revisit resolving logic in replay
also, update documented logic on wiki at:
https://github.com/ikreymer/pywb/wiki/PyWb-Record-Lookup-and-Revisits
support replay of records that have Transfer-Encoding: chunked, but
were not actually rewritten to the warc as chunked.
Attempt to parse chunk length, and if failed, fallback to treating
record as not chunked
Robust chunked data exception handling.