add add --native option to kpm pack which will crossgen all dependencies in the output
Merge pull request #586 from aspnet/packnative

add --native option to kpm pack
Fix external source files reference with "code" pattern in project.json

- Reference external source files with forward slashes on Windows works now
- Reference external source files with backslashes on Mono works now
Upgrade Newtonsoft.Json to 6.0.4 version