Add more codec tests (from Cyphre)

Originally proposed by Richard Smolak in
https://gist.github.com/cyphre/9254624

To not have fixture data modified by Git, add an exception to
gitattributes.
Improve .gitattributes config & comment to be explicit about the effects

The "text" attribute is removed, because it is misleading. It is
basically meaningless with eol=lf. In this combination, `text` does
_not_ lead to auto-detection of text files.
Merge pull request #15 from earl/codec-tests-with-fixtures

Codec tests with fixtures. Thanks, Cyphre and Earl.