1. Add a "card" field to potential class. pot.card tells the cardinality
   of every variable. pot.card[i] equals the number of values of
   pot.variable[i]
2. Denote "card" and "variables" by numpy.ndarray instead of list or
   tuple
3. Complete the demoBurglar
4. Add some utilities:
        AssignmentToIndex.py
        IndexToAssignment.py
        orderpot.py
        potvariables.py
5. Change the directory:
        a) Add "demo" directory to include all demos
        b) Add "test" directory to include the unittest file for the file
            in "brml" and "demo" directories
6. Code added conformes to "PEP8" style guide
7. Reference: programming materials from Daphne Koller's
                PGM class CS228 on coursera
Merge pull request #1 from MingjunZhou/demoBurglar

Add a demoBurglar demo