OPENNLP-1650 Update DownloadUtil to use Models release 1.2

[mawiesne]

Change

• adapts DownloadUtil, related classes and tests towards Models 1.2
• updates index.html in opennlp/tools/util to latest data Models 1.2 for DownloadParserTest
• introduces DownloadUtil.ModelType#LEMMATIZER as related models are now available
• adds LemmatizerModelLoaderIT
• extracts some cnp'ed strings to constants
• fixes broken JavaDoc in PerceptronTrainer along the path

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