Release Name: 0.3.0

Notes:
Interface changing for writing: save is now atomar. Other interface improvements. Documentation completed, new mini tutorial added.


Changes:
- Documentation completed and a Mini Tutorial added
- Interface changes:
  - Test if a value for a tag can be saved is now done in
    MiniExiftool#save
    => There is no check at the moment you set a value:
       the tag occurs in MiniExiftool#changed_values
    => While calling MiniExiftool#save errors can occur (see next point)
  - MiniExiftool#save is a transaction: if one or more error occurs th file is
    not changed! In such a case the errors can be found in MiniExiftool#errors
  - Parameter opts of MiniExiftool.initialize is now a Hash with two options:
    - :numerical => read metadata as numerical values
    - :composite => read also composite tags
- Tests added