addresses issue #60

[rsgalloway]

• fixes bug where changes in frame size (e.g. 9 to 10, 99 to 100, etc) cause incorrect pad
• disables strict padding in lss by default (adds --strict option)
• updated tests
• bumps version to 0.5.3

before:

>>> pyseq.strict_pad = False
>>> items = [
...     'file.7.jpg', 'file.8.jpg', 'file.9.jpg',\
...     'file.10.jpg', 'file.11.jpg', 'file.12.jpg',\
...     'file.87.jpg', 'file.99.jpg', 'file.111.jpg'\
... ]
>>> seq = pyseq.get_sequences(items)[0]
>>> seq._get_padding()
'%02d'

after:

>>> seq = pyseq.get_sequences(items)[0]
>>> seq._get_padding()
'%d'

lss before (120 unpadded frames):

   9 test.%d.exr [1-9]
  90 test.%02d.exr [10-99]
  21 test.%03d.exr [100-120]

lss after:

 120 test.%d.exr [1-120]