Refactor to remove 'src' directory
DIFFEO-94: use yakonfig in streamcorpus_pipeline

Every pipeline stage now derives from ...stages.Configured, which finds
the config_name in the global configuration and stuffs in a self.config.
streamcorpus_pipeline is itself a top-level configured item.
Add --dump-config option

streamcorpus_pipeline --dump-config full prints the entire current config;
streamcorpus_pipeline --dump-config effective prints diffs vs. the default
config
Make pytest-diffeo >= 0.1.4 dependency explicit

for redis_address fixture
DIFFEO-53: add to_kvlayer index (docid,time)->source

Also make the indexes configurable on to_kvlayer
Merge branch 'switch-to-instance-collection'