Make behavior of getStatesForPipelineId more consistent

It now returns an unmodifiable list. Earlier, if the state existed, it
would pass a direct reference to the list which could be modified by
the callee. If not, it would create an empty state list but not store
its reference in the data map.

Additionally, upgrade guava version to 15.0
Merge branch 'master' of https://github.com/Sanrag/groningen

Conflicts:
	src/main/java/org/arbeitspferde/groningen/historydatastore/MemoryHistoryDatastore.java