use CMAKE_PREFIX_PATH instead of CMAKE_FILE_ROOT_PATH; removed the
requirement to use EXACT version numbers for dependencies, cleanup
Merge branch 'build_enhancement' of https://github.com/sveneh/cpp-ethereum into build_enhancement

Conflicts:
	cmake/EthDependencies.cmake
	extdep/CMakeLists.txt
	extdep/json-rpc-cpp.cmake
Merge branch 'build_enhancement' of https://github.com/debris/cpp-ethereum into build_enhancement