Update _glyph_functions.py

Solving issue to be able to pass columns other than 'x' and 'y' in bokeh.plotting in bokeh-dev-0.5.2

e.g:
```python
source = ColumnDataSource(data=dict(
    p = np.random.random(N),
    q = np.random.random(N)    
))

square('p','q',source=source)
show()
```