Added support for inserting clef on the middle & end of a Stave.

For supporting clef on the end of stave:
I extended Stave class to add glyph on the end of the Stave by adding end_glyphs variable.

And middle of stave:
Added new class ClefNote which inherits Note.
![2014-02-20 16 45 14](https://f.cloud.github.com/assets/2025065/2216118/f3b71b30-9a02-11e3-970c-0de292ddb4f3.png)