+ replaced several functions by template magic

- removed IDE leftovers
- adjusted const’ness of some functions
- style guide

- to_string() method is now called dump()
- syntax borrowed from Python’s json.dumps()

- fixed issue #9
- also removed std::mutex member variable
- also added “std::” prefix to size_t variables

If the new operator throws in the json::find methods then result.oi_ is
deleted again in the destructor of json::iterator/json::const_iterator,
which is a double delete and undefined behaviour.