Expand Label to surface using patch info

Following up on issue https://github.com/mne-tools/mne-python/issues/1180, adds the following modifications:

 - in the `Label` constructor, add the option to expand a label from a source space to the entire surface (using patch info)
 - add a `Label.fill()` method to expand a label defined in source space to the surface
 - adds to `stc_to_label` the option to expand the label using patch info
 - adds to `setup_source_space()` the `add_dist` option to call `add_source_space_distances` on the source space before returning it, with a warning that the default is going to change to True in 0.9 
Now I get this error:
```
======================================================================
FAIL: Test stc_to_label
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/larsoner/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/larsoner/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/numpy/testing/decorators.py", line 146, in skipper_func
    return f(*args, **kwargs)
  File "/Users/larsoner/custombuilds/mne-python/mne/utils.py", line 607, in dec
    ret = function(*args, **kwargs)
  File "/Users/larsoner/custombuilds/mne-python/mne/tests/test_label.py", line 420, in test_stc_to_label
    assert_equal(len(w), 2)
AssertionError: 1 != 2
    '1 != 2' = '%s != %s' % (safe_repr(1), safe_repr(2))
    '1 != 2' = self._formatMessage('1 != 2', '1 != 2')
>>  raise self.failureException('1 != 2')
```