Implemented OpenDKIM php extension signing.
feature #285 Implemented OpenDKIM php extension signing. (xdecock)

This PR was squashed before being merged into the 5.1-dev branch (closes #285).

Discussion
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Implemented OpenDKIM php extension signing.

Based on the libopendkim extension available on my github

: https://github.com/xdecock/php-opendkim

I'll need some extensive testing before considering stable, but should be a drop-in replacement of DKIMSigner. the test will more than probably not work on every machine based on local settings, i'll be glad to handle bugfixes.

Thanks for your helps

Commits
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4469bfa Implemented OpenDKIM php extension signing.