Update Project Settings and improved Obj-C API

[q231950]

This pull request:

• updates the project to the recommended project settings (Xcode Version 9.2 (9C40b)) https://github.com/q231950/HexColors/blob/d8acddd4ad2f6ec1e9160ebcb7e4236e5c02cebb/Sources/HexColors.swift#L27-L35- improves Objective-C interoperability [@objc/@nonobjc]()
• makes the compiler happier error due to compile time complexity

[codecov-io]

Codecov Report

Merging #49 into master will decrease coverage by 2.97%. The diff coverage is 62.5%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage     100%   97.02%   -2.98%     
==========================================
  Files           2        2              
  Lines         175      235      +60     
  Branches        0       10      +10     
==========================================
+ Hits          175      228      +53     
- Misses          0        7       +7

Impacted Files	Coverage Δ
Sources/HexColors.swift	94.01% <62.5%> (-5.99%)	:arrow_down:
HexColors-iOSTests/HexColors_iOSTests.swift	100% <0%> (ø)	:arrow_up:

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[q231950]

@mRs- I'll have a look into adding some tests for the obj-c initialisers.