Fix bad ref count dec in Python APIs font index/contains

`fontforge/python.c` had a few places where `PYGETSTR()`/`ENDPYGETSTR()` defines were not used correctly, and worse, a couple places where `Py_DECREF()` was used on the wrong object rather than correctly using `ENDPYGETSTR()`.

When building FontForge with Python3 and running Python scripts, this could result in active variables being garbage-collected out from under running code.  Oh, and leaking memory.  And FF crashes.  This only happened with Python3, not Python2.

Test failures and FF crashes could be demonstrated when using the construct
```
    glyph = font1[glyphName]
```
as the problems in `PyFF_FontIndex()` were a bit worse.

Failures couldn't be demonstrated as easily when using the construct
```
    if glyphName in font1:
```
though the code in `PyFF_FontContains()` was similarly wrong.

It would seem as though this code was done before the PYGETSTR/ENDPYGETSTR ideas were fully worked out?  As part of enabling the code for Python3 compatibility.  Certainly fully using those defines makes the code shorter.

This change has been tested under Python 2.7.6 and Python 3.4.0 (Ubuntu):
* with Python2 w/o fix - tests succeed
* with Python3 w/o fix - gross failures and "Segmentation fault (core dumped)"
* with Python3 w/fix   - success
* with Python2 w/fix   - success

A test script is also added, `test1008.py`, that tests a number of interrelated APIs for scanning all glyphs in a font:
```
  # Checking these API operations from python.c:
  #
  #   if integer in f:          - font has a glyph for this encoding value?
  #   glyph = f[integer]        - get glyph object for encoding value index
  #   for name in f:            - iterate through glyph names in font
  #   glyph = f[name]           - get glyph object for glyph name key
  #   for glyph in f.glyphs()
  #                             - iterate thru glyphs in GID order
  #   for glyph in f.glyphs('encoding')
  #                             - iterate thru glyphs in encoding order
  #   if name in f:             - font has a glyph for this glyph name?
```
I haven't fully checked out all usages of `PYGETSTR`/`ENDPYGETSTR` in python.c, and routines `GlyphsFromTuple()` and maybe `PyFF_PrivateIndexAssign()` look needy.  Nor have I checked on all uses of `Py_DECREF()`, just enough to say not misused a lot.