Refactored logic of --verify-all option

Check if there are no problem files before attempting to loop over them.
Added problem resources to repository

201_laserbeam.gif was renamed to 202_laserbeam.gif to reflect the fact
that the image is referenced in problem 202, not problem 201.
Updated problems.txt to reflect local resources

Removed links to Project Euler website since the resources are now
stored in the EulerPy package anyways.
Added copy_resources method to Problem class
Updated guidelines in CONTRIBUTING.rst