Change multi-byte write to use slice notation

[caternuson]

As mentioned in #25, this library currently allows writing multiple memory locations by using the syntax fram[0] = [0,1,2,3] which behaves inconsistently from other Python usage, where this would attempt to store [0,1,2,3] in position [0] of fram. This should be changed to something more like fram[0:3] = [0,1,2,3] to use slice notation.

The built in bytearray provides a good reference example:

>>> foo = bytearray(4)
>>> foo[0] = [1,2,3,4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'list' object cannot be interpreted as an integer
>>> foo[0:3] = [1,2,3,4]
>>> foo
bytearray(b'\x01\x02\x03\x04\x00')
>>>