[#78] Fix panic on start of server.

This was caused by a leftover creation of an actor system after the move
to the newer version of Actix-Web.

Prune deps and split out integration tests.

Remove redundant string conversion

Use macro derives to clean-up handler boilerplate.

Macro derives are provided, via proc macros, by actix for `Message` and
`MessageResponse` which means we don't need _any_ of the duplicated
boilerplate that was there! Awesome!

Fix all the breaking changes from Actix Web v1.0.0.

There were significant breaking changes described in the migration guide
at https://github.com/actix/actix-web/blob/master/MIGRATION.md#100 — and
a lot that weren't documented anywhere!

Some notable ones are:

* A new way to get at query and path params (which is much nicer,
admitedly.
* A completely new way to specify routes and middleware — the DSL is
basically entirely overhauled.
* Removal of the `TestServer` helpers from `actix-web` means you now
need to test through `actix-http-test` instead.
* We are now able to assert on the body of failed requests, meaning the
tests can check the actual error responses on those too.

Update git2 requirement from ^0.8.0 to ^0.9.1

Updates the requirements on [git2](https://github.com/rust-lang/git2-rs) to permit the latest version.
- [Release notes](https://github.com/rust-lang/git2-rs/releases)
- [Commits](rust-lang/git2-rs@0.8.0...0.9.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Block docker image with osx

Build and push Docker containers on tags.

Make it possible to build and push Docker containers when a release
build happens. The Docker script now copies the binary from locally
instead of fetching from GitHub releases in order to avoid a
race-condition — and is completely automated.