Simplified metadata collection to make it less automatic and easier to understand by reading class declarations.
- Changed CommandAttribute from a metadata-only attribute to an ExportAttribute that specifies Command contract type.
- Changed CommandExportConventions to only supply CommandType value of CommandAttribute instances and nothing more.
- Change Help command to display usage section.
- Changed MefBuild.exe to print help information when no command-line arguments were supplied.
- Changed Help command to display only common commands (for now, commands with summary).
- Created ParameterAttribute to supply metadata describing command parameters.
- Renamed Help to Usage to free up the name for a new command type.