Add count member function #45

A typical operation is to find if a json object has a given key, this is typically done like:

json j;
bool has_key = j.find("key") != j.end();

The standard containers (std::map, std::set,...) offer a count member function that counts the number of occurrences of a key, such that one can:

std::map<std::string, int> m;
bool has_key = m.count("key");  // -> count == 0? false : true

It would be nice to have something similar since this is a very common operation.