SLICE-8 Rework mapper initialization
Merge branch 'SLICE-8' of https://github.com/Cognifide/Slice into SLICE-8

Conflicts:
	slice-mapper/src/main/java/com/cognifide/slice/mapper/GenericSlingMapper.java
	slice-mapper/src/main/java/com/cognifide/slice/mapper/SlingMapperBuilder.java