#288 Fixed incorrect parse results from mapping entries split across newlines

[fktn-k]

As reported in the issue #288, the the parser & lexer implementations didn't cover mapping entries split across newlines.
To fix that, the handling of key separators (:) have been moved from the lexer to the deserializer, and defer checking the existence of a key separator followed by a node associated to a block mapping key.
To validate the above changes, the test suite has been updated with some new test cases.

---

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 21e7f900ddea9d3a3fabd871f79d4b3f33a4c18a on bugfix/288_parse_mapping_entries_split_across_newlines into d77e7f58b68970b6a12c67dde6e2569863bc7328 on develop.