Sitemaps parser

See #38 

This adds a SiteMapParserBolt and a test class. It also reorganizes the test resources and makes the ParsingTester more generic.

As discussed, this Bolt uses a non-default stream 'status' to output the newly discovered URLs and the default stream for documents that are not marked as being sitemaps.

Could you please review this? Thanks!
I'd assumed that the handling of the fails should be done within the spouts. The 'external' resources would extend the default components where available e.g. Kafka's spout to allow keeping track of how many times it already failed, for instance by adding a custom key value in the metadata. But there could be a separate stream as you suggested. The whole thing would get a bit more complex (I know, I like to keep the library and the assumptions that go with its use as lean as possible). Let's keep that in mind and tack it in a separate discussion.

Shall we assume for now that : 
a) the errors will be reproduced every time we run the parser on the same content == always ack? 
or
b) there could be something in the Fetching step that could vary from one run to the other (e.g. server returning partial content) which would justify giving the same URL another try?