Fix #132:  enforce 64-bit ranges on long values for protobuf attrs.

See #132.
```python
>>> INT64(sys.maxint)
9223372036854775807L
>>> INT64(sys.maxint + 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Python/2.7/site-packages/google/protobuf/internal/type_checkers.py", line 119, in CheckValue
    raise ValueError('Value out of range: %d' % proposed_value)
ValueError: Value out of range: 9223372036854775808
>>> INT64(-sys.maxint - 1)
-9223372036854775808L
>>> INT64(-sys.maxint - 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Python/2.7/site-packages/google/protobuf/internal/type_checkers.py", line 119, in CheckValue
    raise ValueError('Value out of range: %d' % proposed_value)
ValueError: Value out of range: -9223372036854775809
```