Use os.pathsep to sepatate multiple ImageCollection load_pattern

The colon `:` does not work on Windows as it can be part of a path.

Fixes the following test error on Windows:
```
======================================================================
ERROR: test_null.test_null_imread_collection
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python34\lib\site-packages\nose\case.py", line 198, in runTest
    self.test(*self.arg)
  File "X:\Python34\lib\site-packages\nose\tools\nontrivial.py", line 60, in newfunc
    func(*arg, **kw)
  File "X:\Python34\lib\site-packages\skimage\io\tests\test_null.py", line 45, in test_null_imread_collection
    collection[0]
  File "X:\Python34\lib\site-packages\skimage\io\collection.py", line 355, in __getitem__
    n = self._check_imgnum(n)
  File "X:\Python34\lib\site-packages\skimage\io\collection.py", line 389, in _check_imgnum
    % num)
IndexError: There are only 0 images in the collection
```