0.3.0:

* Context:
  - Change mask_surface() to mask() and
    set_source_surface() to set_source().
  - Add typedef for Matrix, but a real Matrix 
  class would be nice.
* Pattern: Created hierarchy of classes.
* Check for errors in constructors.
* Exception: Rename to logic_error, and throw 
  std::bad_alloc or std::io_base::failure instead 
  when appropriate.
  (Murray Cumming)

