update tolvera/draw/ruler.py

[kanakOS01]

closes #3

Changes made:

• Separate the draw() method into 2 methods - draw_lines() and draw_labels() as tv.px.stamp() was raising an error when called from a ti.kernel.
• The ticks on the x axis were not visible as they too were being drawn in positive x direction so I added an attribute to store if the axis was x or y and then draw the ticks accordingly.