Adds features: variable degree range, tap in center to close menu, simpler buttons

I modified LIVBubbleMenu for my project and decided to clean it up and make a pull request.

Changes:

- my editor automatically fixed all of the trailing whitespace
- variable bubble degree range
    - `_bubbleStartAngle` determines where the bubbles will start
    - `_bubbleTotalAngle` determines how many degrees to rotate through
    - for example,  set them to 0 and 180, respectively to show bubbles on the bottom half of the circle (for some reason, degree values run clockwise instead of counter clickwise)
    - there are some improvements to be made for this, but it's an easy start, and all I needed for my project
- tap in center to close menu
    - added `touchesBegan` handler to hide menu if the user taps anywhere inside the bubble ring
- easy buttons
    - existing button method doesn't scale images correctly, so I made an `easyButtons` variable that switches between your method (better for views with alpha channels) and the simpler method of just assigning the image to the button