PersistentMap: untested SKV backend

Conflicts:
	CMake/FindPackages.cmake
	CMake/FindRequired.cmake
Servus: add API for continuous browsing
Add unit test

Conflicts:
	.gitexternals
	CMake/FindRequired.cmake
Fix travis
Merge pull request #131 from eile/master

Servus: add API for continuous browsing, SKV backend for PersistentMap