Reorganise 'Source' and 'Test' directories

[danielrbrowne]

This PR resolves #167:

• Reorganises the Sources and Tests directories to make navigating the project in Xcode much quicker and easier.

[codecov-io]

Codecov Report

Merging #168 (04bd673) into master (de88b30) will decrease coverage by 0.44%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
- Coverage   89.95%   89.50%   -0.45%     
==========================================
  Files          57       57              
  Lines        2916     2916              
==========================================
- Hits         2623     2610      -13     
- Misses        293      306      +13     

Impacted Files	Coverage Δ
Sources/Extensions/Array+CalculateMD5Hash.swift	100.00% <ø> (ø)
Sources/Extensions/Encodable+Encode.swift	100.00% <ø> (ø)
Sources/Extensions/String+HexStringToData.swift	91.66% <ø> (ø)
Sources/Helpers/DeviceTokenHelper.swift	92.30% <ø> (ø)
Sources/Models/AuthData.swift	100.00% <ø> (ø)
Sources/Models/InstanceId.swift	100.00% <ø> (ø)
Sources/Models/Metadata.swift	100.00% <ø> (ø)
Sources/Models/PersistenceConstants.swift	100.00% <ø> (ø)
Sources/Models/PublishId.swift	100.00% <ø> (ø)
Sources/Models/PushNotificationsAPIError.swift	83.33% <ø> (ø)
... and 47 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update de88b30...04bd673. Read the comment docs.

[TomKemp]

I don't know this project well enough to comment on whether this new structure makes sense for the project, but generally the directory names look reasonable 👍