Feature Swift Package Manager support

[zenangst]

• Add support for Swift Package Manager
• Sync package version with CocoaPods
• Remove Xcode project
• Update travis configuration
• Update gitignore

[codecov-io]

Codecov Report

Merging #207 (1fafde6) into master (b56a672) will decrease coverage by 8.62%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   79.49%   70.86%   -8.63%     
==========================================
  Files          12        5       -7     
  Lines         863      127     -736     
==========================================
- Hits          686       90     -596     
+ Misses        177       37     -140     

Flag	Coverage Δ
iOS	70.07% <ø> (ø)
macOS	56.69% <ø> (-20.95%)	⬇️
tvOS	33.85% <ø> (ø)

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