changed PrefixTree to StringMap

- Removed Holumbus/Data/PrefixTree
- Added dependencies to data-stringmap
changed PrefixTree to StringMap

- Removed Holumbus/Data/PrefixTree
    - Added dependencies to data-stringmap
Merge branch 'master' of github.com:UlfS/holumbus

Conflicts:
	searchengine/src/Holumbus/Index/ComprPrefixTreeIndex.hs
	searchengine/src/Holumbus/Index/PrefixTreeIndex.hs
	searchengine/test/Index.hs
	searchengine/test/Interpreter.hs