Added the snapPoint property, increased performance, and fixed bugs

Fixes

• Addressed issue #94: Too much widget rebuild occurring when the user slides the panel. This fix results in huge performance benefits when using the panelChild and panelBuilder properties
• Addressed issues #102, #111: Fixed issue where tapping on the panel closes it (when using the panelBuilder)
• Addressed issues #24, #70, #108, #121: Changed backdropTapClosesPanel to use onTap and onVerticalDragEnd so swipes on the backdrop cause the panel to close

Features

• Addressed issue #107: Added duration and curve properties to the PanelController's animatePanelToPosition method
• Addressed issues #12,#77,#86,#100: Added a snapPoint property to allow the panel to snap to a position midway during its slide

Documentation

• Updated documentation to reflect new features and fixes
• Updated copyright year in the LICENSE