YDK release 0.8.4

YDK release 0.8.4

New features and enhancements

- Expanded meta data in Python bundles by adding flags has_must and has_when (#927)
- Added utility function to compare two entities and build diff (#925)
- Adjusted C++ code to work with gcc compiler version 4.8.5 (default for CentOS-7.x)

Resolved GitHub issues

- 'ietf-netconf-monitoring:get-schema' returns schema with escape sequences (#614)
- NETCONF provider should raise more appropriate exceptions (#774)
- YList test fails when list entity added before keys are initialized (#800)
- Inherited classes not initialized properly in Python3 when constructor uses super() (#921)
- Delete operation fails on container (#931)
- Absolute path for generated entity of second level list child is incorrect (#933)
- Go: When entity having children is built from model API the child entities parent is nil (#938)
- gNMI Provider does not preserve original strings (#940)
- gNMISession fails to decode GetRequest response when returned values is empty string (#943)
- No way to retrieve numeric value of enums in cpp generated code (#944)
- XmlSubtreeCodec fails decode payload with augmented component (#956)
- Debian package for C++ bundles getting installed to wrong location (#962)

Documentation improvements

- Added documentation for XmlSubtreeCodec and JsonSubtreeCodec.

Bundle improvements

- Released cisco-nx-os bundle to support Cisco NX OS 9.3.1
- Released cisco-ios-xr bundle to support Cisco IOS XR OS 6.6.3