Refactor code to better implement C++11 best practices

[aarontay]

Per feedback from @tomaszkapela on PR#46, recommended C++11 updates that fall out of scope of PR#46 can be implemented as follows:

src/snap/config.cc:

74    std::string Config::get_string(const std::string& key) const {
75        auto str_map = rpc_map.stringmap();
76        return str_map.at(key);
77    }

--Oneline this method as well as all the others

src/snap/config.h:

24    /**
25    * A Rule of type std::string
26    */
27    typedef Rule<rpc::StringPolicy, rpc::StringRule, std::string> StringRule;

--Since C++11, it's preferred to write using StringRule = Rule<...

53    template<class P, class R, typename T>
54    class Rule final : public P {
55    private:
56        class rule final : public R {

--Consider new naming choice for Rule -> rule. Maybe internal_rule or rule_impl

90    Rule(const std::string& key, bool req) {
91        rule rl(req);
92       auto rule_ptr = this->mutable_rules();
93       (*rule_ptr)[key] = rl;

--(*rule_ptr)[key] = rule(req); to avoid triggering a move assignment

Please consider providing additional refactor requests in comments