Swift Package Manager Support

[lightsprint09]

Support for SPM

• Changes file structure to support SPM

• Adds Package file

• Adds 2 new Travis targets (SPM on Mac and Linux)

• Removes usage of NSNull, NSObject & NSDictionary - Please check if this is ok.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7088d80). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #76   +/-   ##
=========================================
  Coverage          ?   56.36%           
=========================================
  Files             ?       26           
  Lines             ?      809           
  Branches          ?        0           
=========================================
  Hits              ?      456           
  Misses            ?      353           
  Partials          ?        0

Impacted Files	Coverage Δ
Tests/JSONCodableTests/ClassInheritance.swift	73.68% <ø> (ø)
Tests/JSONCodableTests/ClassInheritanceTests.swift	86.66% <ø> (ø)
Tests/JSONCodableTests/EncodeNestingTests.swift	57.14% <ø> (ø)
Tests/JSONCodableTests/NestItem.swift	77.77% <ø> (ø)
Source/JSONHelpers.swift	31.25% <ø> (ø)
Source/JSONEncodable.swift	50% <ø> (ø)
Tests/JSONCodableTests/HelperTests.swift	100% <ø> (ø)
Tests/JSONCodableTests/UtilityTests.swift	100% <ø> (ø)
Tests/JSONCodableTests/Company.swift	100% <ø> (ø)
Source/JSONEncodable+Mirror.swift	100% <ø> (ø)
... and 16 more

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