CI should allow for publication of NuGet package.

[austinlparker]

Currently, the process for publishing a new release goes something like this:

• PR is merged to master
• Circle performs a build/test/pack workflow which outputs a nupkg file.
• This file is downloaded then uploaded through https://nuget.org web interface.

The following would be more desirable:

• PR is merged to master
• When we want to do a release, master is tagged (with a semver) and pushed.
• Circle runs a tag build, where this tag is pulled in as an environment variable.
• The tag is set as the version of the assembly, nupkg, and tracer version tag reported to LightStep.
• Once the package has been generated, it is uploaded to nuget.org via API.

[austinlparker]

This is blocked by libgit2/libgit2sharp#1637.

[austinlparker]

Given that I haven't seen any movement on the issue I opened in that other repo, I refactored this to use a git tag via CI and some manual string janitoring to create the version string.