Add dumper module to convert Ruby hash => Toml string

As requested on #14.

Based on @jm / @dirk solution.
Update gem version to 0.1.2
Removes if in the parser using double dispatch.
Makes symbolize_keys instance variable so it doesnt goes around between messages
Less LOCs FTW! Because why not?
Renames double-dispatch methods to make Visitor pattern more explicit.
Merge pull request #19 from tonchis/refactor_parser_double_dispatch

Removes if in the parser using double dispatch.
Update Dumper interface
Update README.md examples.

Show the return value and how `dump` method works