added "strict" option to b64decode

Argument for padding character.
Argument to specify "strict" decoding or not (default: Yes)
- if strict=True, exception like base64 module on bad input
- if strict=False, try the following to "fix" the input:
  1. add pad byte(s) to end
  2. remove non-alphabet bytes at end
  3. remove 1-4 bytes at end (to make strlen % 4 == 0)
  4. remove non-alphabet bytes at beginning