Removed textbuffer "changed" event handling.

Pressing the return key is now caught by the key_pressed event of the TextView. As a result, commands can now be executed by pressing return anywhere in the input.
Added 'font' property to set font family and font size of the interpreter.
Added property 'margins' to set the left and right text margin.
The Home/End key now jumps to the start/end of the input line.
Added signals that are emitted on stdout/stderr write operations.
Updated README.
Updated README.
Typo in README.
Updated README.
Implemented tab completion.
Tab complete now gets updated locals everytime completion is started.
Only add tag 'protected' to the output buffer if it does not exist.
It's now possible to change the error text color during runtime.
Added method GtInterpreterStandardOutput.write_pixbuf()
Added method GtkInterpreterStandardOutput.write_image(filename)
Implemented basic matplotlib aware version of the widget.
Moved matplotlib shell to demo directory.
Added options for plot figsize and dpi.
Changed matplotlib demo banner.
Updated README.