cleanup build file. Start some modularisation.
Merge branch 'develop' of https://github.com/ethereum/cpp-ethereum into build_enhancement
refactor C++11 detection, BuildFile.h generation
refactored external library detection

TODO this is not proper CMake way, needs further refactoring
Merge pull request #367 from sveneh/build_enhancement

CMake modularisation
Merge branch 'develop' into ethereumx