Add initial support for Nagios Performance Data

[atc0005]

• Add PerformanceData type to represent a single, complete Performance Data entry

• Extend ExitState type to hold Performance Data entries

• Modify ExitState.ReturnCheckResults() method to process Performance Data entries if present

• Add ExitState.AddPerfData() method with optional flag to skip validation

  • intended to allow client code full control of validation

This is a first draft and has not undergone sufficient testing to be considered stable.

Documentation updates have not been made yet to illustrate use of the new functionality.

refs GH-81