String conversion to binary array

NuPIC Network API lacks of a mechanism to read binary arrays from a record and pass them to  PassThroughEncoder. The problem is that FileRecordStream only accepts 'string', 'datetime', 'int', 'float', 'bool' for record data types in CSV files.

This PR brings a simple solution to this. We simply use `sdr` as data-type for the binary array:

```
Square
sdr
T
000000000000000000000000000000000110000000000000011000000000000000000000000000000000000000000000
000000000000000000000000000000000000110000000000000011000000000000000000000000000000000000000000
000000000000000000000000000000000000000110000000000000011000000000000000000000000000000000000000
000000000000000000000000000000000000000000110000000000000011000000000000000000000000000000000000
000000000000000000000000000000000000000000000110000000000000011000000000000000000000000000000000
```

Python loops are slow, especially with multiple comparisons.  Consider using a list comprehension:

```python
def parseSdr(s, _allowed=set("01")):
  """Parses a string containing only 0's and 1's and return a Python list object.
  """
  assert isinstance(s, basestring)
  sdr = [int(x) for x in s if x in _allowed]
  if len(sdr) != len(s):
    raise ValueError("The provided string %s is malformed. The string should "
                     "have only 0's and 1's.")
  return sdr
```