Update Nuget deploy key.

Merge pull request #50 from adriangodong/update-nuget-key

Refresh nuget API key.

Update README.md

Merge pull request #4 from adriangodong/test-for-string-source

Added unit test for StringPrivateKeySource

Merge pull request #2 from adriangodong/downversion-to-netcoreapp1.1

Downgrade library target to netstandard1.4

Initial code

Initial commit