Allow 'Z' for no timezone

The spec actually says that "an uppercase 'Z' character MUST be
present in the absence of a numeric time zone offset".
Describe better the exceptions that the parsing can raise
Make the parsing of dates specific to Atom and RSS2

Atom and RSS2 do not expect dates to be presented in the same way.
Parse correctly <rss> top-level tag of RSS2
Merge pull request #18 from Chris00/master

'Z' when no timezone + RSS2 parsing