Update nuspec files for all class libraries

[nfbot]

Need to add project logo as package icon and add a new variable in the description.

• add project logo
• update license to MIT
• update owners and authors
• add native checksum variable

Basically follow this PR: nanoframework/nanoFramework.Networking.Sntp#82

[networkfusion]

Since nanoframework.Json only required a change of license, this was committed directly.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[josesimoes]

In progress!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[networkfusion]

In progress!