New function TomlParser::close()

Function to explicitely close the std::ifstream source_file_ (input file).
After TomlParser::parse() is done, before returning the doc object, close the file.

This is mostly required on Windows due to
the fact that when a file is opened it is locked from suppression/renaming
by other processes. If a TomlParser object is not freed then the toml file
becomes actually locked.

To prevent this, explicitely close the toml file after beeing read/parsed.
Call this->open() instead of open() for better clarity.

What is being called is TomlParser::open(), not std::fstream::open(). Kill
any possible ambiguity.
Merge pull request #12 from nbigaouette/close

Close file