The Register class currently can't handle setting the `value'
attribute. Fix this. Also add a `zero' convenience method for zeroing
out a register.

After adding the following doctest:

    >>> abc.value = 0
    >>> abc.value
    0

but before fixing __setattr__, I get:

    **********************************************************************
    File "register.py", line 42, in __main__.Register
    Failed example:
        abc.value = 0
    Exception raised:
        Traceback (most recent call last):
          File "/usr/lib/python2.7/doctest.py", line 1289, in __run
            compileflags, 1) in test.globs
          File "<doctest __main__.Register[10]>", line 1, in <module>
            abc.value = 0
          File "register.py", line 106, in __setattr__
            raise AttributeError
        AttributeError
    **********************************************************************
    File "register.py", line 43, in __main__.Register
    Failed example:
        abc.value
    Expected:
        0
    Got:
        49
    **********************************************************************
    1 items had failures:
       2 of  20 in __main__.Register
    ***Test Failed*** 2 failures.

After the fix in __setattr__, all tests pass successfully.

Change-Id: I6d39261b139aae0def58f38a4291baad651b10e3