add dnstap module

https://github.com/CZ-NIC/knot-resolver/pull/39 with some commits on top.

added 40 commits

• c9084a1f...3db1e627 - 29 commits from branch master
• 92606af5 - Adding dnstap module
• 6d86e622 - dnstap testing application
• 1d83e1d0 - Fixing structs after rebasing upstream changes
• bfe5d23a - fixing makefile to compile protobuf if dnstap is enabled
• 9a00f454 - adding dnstap dependencies to bootstrap
• 6e0f9f45 - dnstap tests requires go 1.5+
• afdd1573 - adding dnstap to documentation
• b682370e - minor changes from https://github.com/CZ-NIC/knot-resolver/pull/39
• b5851775 - dnstap.proto: move from contrib into module
• 429f47bc - modules/dnstap: nitpicks
• 0e62dafd - modules/dnstap: move description into the docs

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added 1 commit

• 598c17cf - dnstap module also needs protobuf-c compiler (protoc-c)

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added 1 commit

• f3c2b039 - modules/dnstap: Change option names to socket_path and log_responses, we don't u…

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added 1 commit

• b2677cbc - Test failing make clean on missing dnstap dependencies

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merged

mentioned in commit dkg/resolver@e6ef2907