Fix issue #16 - 1. If parsing a passed URL, use that URL as the base …

Fix issue #16 - 1. If parsing a passed URL, use that URL as the base feedurl. 2. Allow the user to pass a base feedurl to parseString, parseFile, and parseStream. 3. Failing both of the foregoing, if the feed itself declares a base feedurl, use that and re-resolve the relative URLs that were already parsed (tricky bit, that).