Toolbar item image changes based on extension mode change

[sambuddhabasu]

@smcgregor The toolbar item image now changes based on the extension mode change(on/off) The changes made in this PR are:

• Changed default icon to be shown as toolbar item image.
• Added icons which denote whether extension is on/off
• On changing the mode of the extension(on/off), the toolbar item image changes

Note: The icons added look different from that in the Chrome extension. This is because, Safari extension allows only button contents to be filled in black colour. More about this can be read from https://developer.apple.com/library/safari/documentation/Tools/Conceptual/SafariExtensionGuide/AddingButtonstotheMainSafariToolbar/AddingButtonstotheMainSafariToolbar.html

Fixes #29

[smcgregor]

Do they allow shades of gray? We could potentially lighten the color when it is deactivated.

[sambuddhabasu]

I did try changing the icon to different shades of gray but, on the Safari toolbar, they appear to be of the same colour. So, we need to have a black image with a transparent background. Let me know if the current icons in the PR are okay.

[smcgregor]

How do these look in the browser? I think the text might not look great. How about trying to illustrate it symbolically? Maybe use the check mark and the "x" in the center of the lock icon in white/transparent?

[sambuddhabasu]

@smcgregor Thanks for that! I have changed the icons and they look much better and clearer now in the browser. I have squashed the commits so that the PR can be merged. The look of the icons in the browser can be seen below: