#256 Added specialization of from_node() for std::map with compatible key/value types

[fktn-k]

As described in #256, specialization of from_node() for std::map whose key/value types are both compatible with basic_node.
Test cases related to the specialization have also been added.

---

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
• [x] The code coverage on your branch is 100%.
• [x] The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[coveralls]

coverage: 100.0%. remained the same when pulling b2de650bacd6647ae109c6d342ec4261b0c52072 on feature/256_add_specialization_for_std_map into 024294a41557e58bcb16c6ccef9090a35d20816e on develop.