Address changes between draft and spec

[TcM1911]

This updates the code to be inline with the spec. There are some changes between the draft documents and that final draft that has been addressed. This also include some typos both in documentation and attributes.

Changes:

• Add more types allowed for Campaign.AddTargets
• Change AdditionalHeaderFields for Email to map[string][]string
• Fix typo in File.SizeOfInitializedData attribute.
• Change relationship type from RelationshipTypeAVAnalysisOf to RelationshipTypeAnalysisOf
• Fix typo Process.IntegrityLevel
• Add omitempty tag to ThreatActor and Tool types.
• Fix typo X509.SubjectPublicKeyAlgorithm

[codecov[bot]]

Codecov Report

Merging #38 (ac7a5c7) into master (9d20952) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   98.04%   98.04%           
=======================================
  Files          42       42           
  Lines        1741     1742    +1     
=======================================
+ Hits         1707     1708    +1     
  Misses         17       17           
  Partials       17       17           

Impacted Files	Coverage Δ
email.go	100.00% <ø> (ø)
file.go	79.59% <ø> (ø)
meta.go	95.83% <ø> (ø)
process.go	100.00% <ø> (ø)
relationship.go	100.00% <ø> (ø)
threat-actor.go	100.00% <ø> (ø)
tool.go	100.00% <ø> (ø)
x509.go	100.00% <ø> (ø)
campaign.go	100.00% <100.00%> (ø)
malware-analysis.go	100.00% <100.00%> (ø)
... and 2 more

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