Shell escape handlebars file path

Hello @toranb! The change in this PR fixes error with strange characters in the `HANDLEBARS_PATH` and without this change `django-compress` fails to parse the `abspath` for directories with spaces with the following traceback.

```
Internal Server Error: /
Traceback (most recent call last):
  File "/home/user/venvs/django_ember_env/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 137, in get_response
    response = response.render()
  File "/home/user/venvs/django_ember_env/local/lib/python2.7/site-packages/django/template/response.py", line 105, in render
    self.content = self.rendered_content
  File "/home/user/venvs/django_ember_env/local/lib/python2.7/site-packages/django/template/response.py", line 82, in rendered_content
    content = template.render(context)
  File "/home/user/venvs/django_ember_env/local/lib/python2.7/site-packages/django/template/base.py", line 140, in render
    return self._render(context)
  File "/home/user/venvs/django_ember_env/local/lib/python2.7/site-packages/django/template/base.py", line 134, in _render
    return self.nodelist.render(context)
  File "/home/user/venvs/django_ember_env/local/lib/python2.7/site-packages/django/template/base.py", line 840, in render
    bit = self.render_node(node, context)
  File "/home/user/venvs/django_ember_env/local/lib/python2.7/site-packages/django/template/debug.py", line 78, in render_node
    return node.render(context)
  File "/home/user/dir with spaces/emberjs/reference/django-compressor-develop/compressor/templatetags/compress.py", line 149, in render
    return self.render_compressed(context, self.kind, self.mode, forced=forced)
  File "/home/user/dir with spaces/emberjs/reference/django-compressor-develop/compressor/templatetags/compress.py", line 108, in render_compressed
    rendered_output = self.render_output(compressor, mode, forced=forced)
  File "/home/user/dir with spaces/emberjs/reference/django-compressor-develop/compressor/templatetags/compress.py", line 121, in render_output
    return compressor.output(mode, forced=forced)
  File "/home/user/dir with spaces/emberjs/reference/django-compressor-develop/compressor/base.py", line 297, in output
    output = '\n'.join(self.filter_input(forced))
  File "/home/user/dir with spaces/emberjs/reference/django-compressor-develop/compressor/base.py", line 228, in filter_input
    for hunk in self.hunks(forced):
  File "/home/user/dir with spaces/emberjs/reference/django-compressor-develop/compressor/base.py", line 204, in hunks
    precompiled, value = self.precompile(value, **options)
  File "/home/user/dir with spaces/emberjs/reference/django-compressor-develop/compressor/base.py", line 266, in precompile
    return True, filter.input(**kwargs)
  File "/home/user/dir with spaces/emberjs/reference/django-compressor-develop/compressor/filters/base.py", line 194, in input
    raise FilterError(err)
FilterError: /bin/sh: 1: /home/user/dir: not found
```

Thanks for putting this example together!