Files now downloaded from gracaninja
Corrected github URLs, added undo mode in solve.py

python ./solve.py --undo day1 now resets the exercise to the original on the
student branch

Corrected URLS, now they point to gracaninja and use the "raw" file URLs
Merge pull request #27 from ramon-astudillo/student

Corrected github URLs, added undo mode in solve.py