Add UITapGestureRecognizer to clear all drawn lines when the screen is tapped twice.

Also set delayTouchesBegan to YES to prevent dots from being drawn while the system is still waiting to see if a doubleTap begins.
Implemented ability to tap on a line to "select" it.
Added UIMenuController action to delete a selected line.

When a user clicks on a drawn line, an menu pops up to delete it. If the user clicks on delete, we delete the line. If they click outside of the menu, we hide the menu and leave the line in tact.
Implemented a long press gesture.

If a user presses and holds on a line, we select it. When they raise their finger off the screen, we de-select the line.

We'll use this later for moving (panning) the line.
User can now drag and drop lines.

Implemented a pan gesture recognizer to allow users to move lines around. The "selected line" is constantly updated as the move is in process, so that when the move stops, the line has been moved to the new position.