Add Unavailability_MarketDocument_parser and fix ZipParser bug.

[nberliner]

The Unavailability_MarketDocument_parser allows you to parse 4.7.6. Fall-backs (see ENTSO-E API Guide).

I added checks in the ParserFactory to check the document_type for the Unavailability_MarketDocument tag. Please note that I added the document_types for the Outages_MarketDocument_Parser() to the best of my knowledge but cannot give a guarantee.

I further reverted the changes to the ZipParser that were introduced with commit 0145930 . As it was implemented, the ZipParser was handing a list()down to the XMLParser which caused an error. IMHO it is better to handle the occurrence of multiple zip files in the ZipParser directly.