Begin using SemVer release tags

[moorereason]

Proposal

Start tagging releases, beginning with v1.0.0. Cast is stable. We have 100% test coverage with no known defects.

Rationale

As a library, tagging stable releases communicates stability to the user (both in the code base and the API). We should strive to be good citizens of the Go community and help foster a mature ecosystem.

The forthcoming Go dep tool will use SemVer (via Masterminds/semver). Dave Cheney argues two things: tagging releases is a prerequisite to a better ecosystem ("No version information, no tools, and the situation never improves"); and the tag should start with a "v". I care less about the "v", but I prefer to have it.

Tagging releases would also allow us to make larger changes. I'm specifically thinking about spf13/afero #89, which would probably necessitate a new major version.

Aspirations

If we agree that this is a good idea, we should extend it to the other spf13 libraries (afero, cobra, viper, etc). Cast is a tiny library, so I figure we can start here and work our way up.

[bep]

Done.