Publish binaries

[carolynvs]

On tagged builds, 64-bit binaries are uploaded to the cli container on the storage account found in $AZURE_STORAGE_CONNECTION_STRING. I've added this to our CircleCI environment variables (behind an approval step on PRs to prevent leakage).

• I've added a version flag so that people can tell which version they are on:

  $ svc-cat --version
  svc-cat v0.1.0+dev (305ec0c)

• I am uploading a copy of the release to /cli/latest so that we can provide a permanent link in our documentation to download the latest version, for example: https://carolynvs.blob.core.windows.net/cli/latest/Darwin/x86_64/svc-cat.
• Checksums are provided at .sha256 so that downloaded binary can be validated.
• The path segments are based on the results of uname -s (OS) and uname -m (ARCH), which makes it easier for scripting downloads.

The tree structure looks like this:

cli
├── latest
│   ├── Darwin
│   │   └── x86_64
│   │       ├── svc-cat
│   │       └── svc-cat.sha256
│   ├── Linux
│   │   └── x86_64
│   │       ├── svc-cat
│   │       └── svc-cat.sha256
│   └── Windows
│       └── x86_64
│           ├── svc-cat.exe
│           └── svc-cat.exe.sha256
└── VERSION (e.g. v0.1.0)
    ├── Darwin
    │   └── x86_64
    │       ├── svc-cat
    │       └── svc-cat.sha256
    ├── Linux
    │   └── x86_64
    │       ├── svc-cat
    │       └── svc-cat.sha256
    └── Windows
        └── x86_64
            ├── svc-cat.exe
            └── svc-cat.exe.sha256

Closes #13

[arschles]

@carolynvs looks good. I'll merge after CI