Syslog-rfc5424-parser

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- Only install `enum34` as a dependency on older version of Python (thanks Github user pvinci)

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- Switch from PyParsing to Lark for an almost 3x speedup
- No changes to user-facing API
- Add documentation via ReadTheDocs

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- Allow message bodies to contain newlines. If you want to split on newlines, do it yourself up-front. Reported by
GitHub user tfogwill

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- Require `pyparsing` 2.3 or above

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- Pin `pyparsing` to less than version 2.3 until I make this work with the new API for grouping (reported by Github user tfogwill)

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- Properly handle messages with SD pairs that have an empty value (reported by Github user eyalleshem)