A set of Roslyn diagnostics and fixes for Protobuf-net

This will check for common errors when writing classes that are decorated with Protobuf-net attributes for protocol buffers serialization.

At this point it checks for:

• ERROR : Tags set to 0
• ERROR : Tags in the reserved range 19000-19999
• ERROR : Duplicate tags, including collisions with ProtoInclude tags
• ERROR : Inconsistencies between DataMember and ProtoMember tags
• ERROR : Tags set to negative values
• WARNING : DataMember or ProtoMember found on class without DataContract or ProtoContract

Planned features:

• Missing ProtoInclude for derived classes with DataContract / ProtoContract / ProtoMember / DataMember attributes
• Inconsistencies between DataContract / ProtoContract
• Re-number tags on existing ProtoMember / DataMember attributes
• Reorder members according to existing ProtoMember / DataMember tags
• Add tags to properties or fields on existing class marked with *Contract attributes
• Switching from Data* to Proto* tags

The vsix extension is available as an artifact on the Appveyor build.