Release version 0.5.2

• Re-add compatibility for rust versions 1.16.0+, and add CI testing
  with rustc 1.16.0 to ensure this is kept in the future.
• Add some more general documentation updates and clarity increases.
• Add a CHANGELOG.md which mirrors git tag releases.
• Update documentation links to point to docs.rs rather than custom
  hosted documentation.
• Fix ColoredLevelConfig::default being an inherent method rather than
  an implementation of the Default trait.
• Add direct support for the syslog crate under the "syslog-3"
  feature flag.
  • Add a module worth of documentation for using fern with syslog.