Handful of features/improvements

Sorry for piling it on like this, but the commits are cleanly separated.

- Go lint/vet cleanup.
- Ability to change StatsD host/port.
- Replace `maintain_aspect_ratio` with `crop_mode`. The `crop_mode` is also part of the URL API now. First I removed `maintain_aspect_ratio` entirely, but then decided to deprecate it as to not break other people's stuff.
- Support for `border_radius` setting. JSON config and URL API supported.
- Auto-orientation support. This rotates the image according to EXIF orientation data. Most common case are iPhone photos taken in landscape mode.

No rush to review.

Fixes #10