Add test with non-unicode text file

[jaap3]

Added a test that demonstrates the behavior reported in issue #78

Not sure how to fix this, so I'll keep this PR as a draft. Any guidance on how to "fix" this would be much appreciated.

[nedbat]

Can you give me a small example of a Django project using a non-utf8 template?

[jaap3]

Hi Ned. As mentioned in the original issue (#78), in my case it's not actually a template but a changelog.txt file tucked away in a static directory of a submodule. django_coverage_plugin finds it (with the default settings) and stops working because of the encoding error.

As you said:

We should be more robust in the face of these sorts of things.

I'd like django_coverage_plugin to at least continue operating when it encounters a unreadable file. An acceptable solution would be to just mark the file as unreadable/uncovered.

[jaap3]

I've updated the test to more accurately show when/where the issue arises. I've also added a proposed fix.

[jaap3]

Fixed in #83