Support for dead-lettering failed messages

[nfantone]

• Add support for a Dead Letter mechanism. Failing or badly routed messages will now end up in a special dead letter queue instead of being silently discarded.
• Declares new seneca.dlq dead letter queue.
• Declares new seneca.dlx dead letter exchange.
• Changes queue declaration to include x-dead-letter-exchange and x-message-ttl arguments, by default.

This may lead to potential issues if you upgrade from an older version. Please, read this wiki guide.

• seneca is no longer a dependency -> moved to "devDependencies".

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #61 into develop will not change coverage

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