Revert "Merge pull request #18 from incuna/images-fixed"

Restore use of override_settings, with a small fix to its import for
Django 1.4+.

override_settings is the correct way to substitute relative paths for
file:// URLs when rendering locally. Blind regexp matches have too
many false positives to be reliable.

This reverts commit 18db05d5a4e90c65634add9b55d7561b281d747d, reversing
changes made to cf4bd1720b1f6b3cd0fec50752157922796b36eb.

Conflicts:
	wkhtmltopdf/utils.py
	wkhtmltopdf/views.py
PDFTemplateResponse.get_override_settings is easier to override.

Now, PDFTemplateResponse.default_override_settings exists to
dynamically generate the settings to override. This will be documented
in usage.rst.

To support this, pathname2fileurl() is smart enough to figure out
whether a pathname ought to be converted into a file:// URL. Unit
tests for this new implementation now exist.
usage.rst: Much better documentation on using PDFTemplateView

Advice on how to write templates and use context variables.

Guidance on how to use the override_settings() feature to integrate
with other Django apps.

And a workaround for hardcoded paths, in case someone still needs the
broken behaviour that was reverted.
Makefile: make test works again.

Some shells do not like "ENVVAR=foo command", so we use proper Bourne syntax."