v0.3.0

This release most importantly fixes error handling, which was partially broken
in the 0.2 release. Users should upgrade.

• Fix error handling (fixes #2, thanks @winstonewert) and improve documentation
• Make flush calls respect the order relative to write calls
  (5bb26be)
• More extensive documentation
• The crate now relies on the 2018 Rust edition
• Misc. smaller improvements