Add support for parsing perfdata metric strings

[atc0005]

• split performance data related code to separate files
• add new ParsePerfData function to parse a performance data metric string (as emitted by plugins) as PerformanceData values
• add "should succeed" and "should fail" table tests for ParsePerfData function
• add validation support for PerformanceData field values
  • used by ParsePerfData
  • planned use with (nagios.PerformanceData).Validate
• update README to list new performance data metric string parsing support

fixes GH-175