Swift 3.2, SPM Support, and Custom Errors

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Added

• New package file for Elevate to support the Swift Package Manager.
  • Added by Christian Noon in Pull Request #30.

Updated

• The parseEntity behavior on Parser to allow custom Error types to be thrown without converting them into ParserError types.
  • Updated by Dave Camp in Pull Request #29.
• The Xcode project and all schemes to Xcode 9.
  • Updated by Christian Noon in Pull Request #30.
• All source and test code to compile against Swift 3.1 and 3.2.
  • Updated by Christian Noon in Pull Request #30.