Release 6.20.0: 2 new Bundles for Yahoo (OAuth- & API-based now, replacing functionality in c.o.subscribe.crawler)

[changerequest]

2 new bundles:
- c.o.oauth.yahoo
- c.o.subscribe.yahoo

2 new config-files:
- yahoooauth.properties (where the API-Keys need to go)
- yahoosubscribe.properties (enable / disable Yahoo contacts serverwide)

Please note that similar to facebook and msn the API-Keys are domain-specific.

Documentation on how to obtain the API-Keys will be available soon. Until then see : https://developer.apps.yahoo.com/projects

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What will disappear from com.openexchange.subscribe.crawler?

- the crawler-configuration in file c.o.subscribe.crawler/conf/crawlers/yahoo.com.yml
- the line to enable / disable the pld crawler in file c.o.subscribe.crawler/conf/crawler.properties.in
- the accompanying unit-test