[Linter::Analyzer] Initial implementation for checking the keys of the attributes hash

See #88
[Linter::Analyzer] Use the consumer to fetch the values
[Linter::Analyzer] Validate keys of attributes
[Linter::Analyzer] Style
[Linter::Analyzer] Validate platform keys
Merge pull request #94 from CocoaPods/feature-linter-keys-check

[Spec::Linter] Initial implementation for checking the keys of the attributes hash