Enable AT tasks running from examples directory

[lubynets]

1. examples directory brought to more order. Now one can easily add new AT-based tasks by placing the .cpp file into examples directory and adding a proper name in examples/CMakeLists.txt, field EXECUTABLES.
2. Minor cosmetic changes:

• avoid deprecated function Chain::GetBranch() usage;
• provide more precise and informative output in Branch::CheckFrozen() and Branch::CheckMutable() functions.