2013 08 25 process simplest expression

Next TODOs:

- Pass the whole node of source code to metamorphoser_module

    e.g.
    ```ruby
    # Before
    Metamorphoser_module._metamorphose_piece( (hoge( ... )), "hoge", 1, 2 )
    # After
    Metamorphoser_module._metamorphose_piece( (hoge( ... )), "hoge(...)", 1, 2 )
    ```
- Support other cases:
    - operators
    - block
    - other control structures (such as if, begin, etc.)
    - Of course more!
- Restructure internal classes according to the future problems.
- Change Metamorphoser module into class so that it can make many instances to wrap source code more flexibly.