No longer uses background; draws the loaded image itself

Calls Invalidate() to update the image as it's being drawn over
-Unnecessary using directive
Now draws lines between the two points, rather than a square
Using custom pen to draw a thicker line
Now displays/clears the file name as window title
Now invalidates on Close

This causes the screen to be redrawn to clear the background image.
Ctrl+W now closes the current image
Re-enabling the Maximize/Minimize window buttons
+Save menu item with blank handler
+SaveFileDialog we'll use for saving images
Now asking for a file name and saving if accepted
-Unnecessary using directive
Tracks the Save enabledness properly
+ImageExtensions blank static class
+AddLine method to draw a line on an image
Updating line drawing code to now use the helper
-Graphics instance from the form (in helper now)
Merge pull request #3 from alexwnovak/feature/refactor

Feature/refactor
+Tools menu
+Pen menu item with shortcut