Send log level to Papertrail

Currently, every log line sent to Papertrail is prefixed with [<26>](https://github.com/kenperkins/winston-papertrail/blob/35867e421c8d3807060326172920ceb171f7ba85/lib/winston-papertrail.js#L334), which corresponds to the syslog code for facility:daemon and severity:alert. Papertrail allows searching based on these fields, so it would be nice to pass other values.

This patch replaces the static syslog generator with [glossy](https://www.npmjs.com/package/glossy). It's a breaking change, since the log severity now gets passed to Papertrail, and the syslog format now conforms to [RFC 5424](https://tools.ietf.org/html/rfc5424). However, since you can both search and change CSS styling in Papertrail based on log facility/severity, it seemed worth publishing.