Working version of the parser

I made a few changes in the design. Look up ```toml/parser.rb```, ```toml/keyvalue.rb``` and ```toml/keygroup.rb``` files to see the major changes.

Basically I realized that the parser needs to do two things: navigate the key chain and assign a value, so those concerns are handled by ```toml/keygroup.rb``` and ```toml/keyvalue.rb``` respectively. I think that having classes there is an overkill, but we can tidy up the design later.

I'm hungry, gonna eat.

Cheers,
Tonchis