Target for dotnet standard 2.0 and a few small fixes

[yjpark]

First thank for the work, I like farango's design decision a lot since I'm using own encoding for json data.

The changes I've beed added are mainly:

• Target dotnet standard 2.0
• Fix issue with not-ascii characters by adding encoding header
• Fix issue with some encoding issue with JToken with cursor result
• Add some helper to create collection and index (used in scripts to init database schema)