Add support for sk-ecdsa keys

• This release references an updated version of the ssh-key crate which includes support for sk-ecdsa key type
• Minor updates to the API documentation
• The examples/verify.rs tool now prints an informative message on success
• Bump dependencies

Test coverage, dependency updates, docs

Bumping the release version all the way to 0.9.0 to indicate that I'm getting ready to release a 1.0.0 that guarantees API stability. This release brings some additional test coverage, minor code cleanup and updates to the dependencies.

Updated sign() signature

Realised when starting to use this library that passing a reference to a byte slice makes for a more ergonomic API than the
previous Buffer type. Since I haven't released 1.0.0 yet, I'm making this a minor version release even though it is a breaking
change.

v0.3.0

This release includes functionality to ask the ssh-agent to sign arbitrary data. Please see examples/verify.rs for details
on how to use the API.

The API is also changed to take references to keys instead of requiring the caller to create keys that are moved into the calls

Docs and minor cleanup release

v0.2.0

Docs updates, api cleanup

Initial release

This initial public release implements the api to mimic popular ssh-add commands to add, list and remove identities managed
by an agent.