Fix for memory leaks in font handling

[wiredfool]

Fixes #2629 .

Changes proposed in this pull request:

• Fix for memory leak in PILfont getsize
• Fix for memory leak in truetype getsize

[homm]

After return

[wiredfool]

I'm not sure what's up with the Travis errors, as they're non-deterministic. This fails on debian-stretch in one commit or ubuntu-trusty-x86 in another, but the changes should not affect them.

======================================================================
FAIL: TestFontPcf.test_high_characters
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Pillow/Tests/test_font_pcf.py", line 59, in test_high_characters
    self._test_high_characters(message)
  File "/Pillow/Tests/test_font_pcf.py", line 55, in _test_high_characters
    self.assert_image_equal(image, compare)
  File "/Pillow/Tests/helper.py", line 85, in assert_image_equal
    msg or "got size %r, expected %r" % (a.size, b.size))
AssertionError: got size (770, 22), expected (765, 22)

(and when rerun, the ubuntu version passed, and the debian one still failed)

[wiredfool]

This PR now has refactored out a PillowLeakTestCase into helper.py. It's got the current cross platform correct version of the get_mem_usage code from the png leak test case. We should convert the other memory checkers to this helper, so that they use the correct code.