Submenu

[onsah]

Adds the submenu feature described in #10 Some highlights:

• Recursively nested submenus. There is no limit on the number of levels of submenus
• Customizable indicator for submenus
• Automatic creation and removal of submenus when neccessary (Such as inserting the first item or removing the last item in the submenu)

For the details refer to the documentation (mostly changes are in the api section)

Note: This PR should be merged after #47 because it is built on it

[onsah]

@hasanbalci This PR is now ready to be merged.

[onsah]

A few comments for now:

* For example, I have

blue -> light blue
green

and if I move light blue to parent menu by using moveToSubmenu function then submenu indicator still exists even though there is no submenu anymore.

* In the demo, if I click on 'change color' item, I get error in console.

Thanks for noticing this issue. It is caused because I changed moveToSubmenu just detaching from the parent instead of removing it, because when removing a menu item is also destroyed. But this is a problem for moving items that contain a submenu. I am trying to find a good solution.

For the second issue, what kind of error do you get? Can you provide the error because I don't get any error in the console when I click 'change-color'.

Edit: I got the error for the second case. It turns out that I was checking wrong property while determining whether the menu item has click function or not.