[SPARK-11769] [ML] Add save, load to all basic Transformers

[jkbradley]

This excludes Estimators and ones which include Vector and other non-basic types for Params or data. This adds:

• Bucketizer
• DCT
• HashingTF
• Interaction
• NGram
• Normalizer
• OneHotEncoder
• PolynomialExpansion
• QuantileDiscretizer
• RFormula
• SQLTransformer
• StopWordsRemover
• StringIndexer
• Tokenizer
• VectorAssembler
• VectorSlicer

CC: @mengxr

[SparkQA]

Test build #46047 has finished for PR 9755 at commit 13ba079.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class HashingTF(override val uid: String)\n * class Normalizer(override val uid: String)\n * class SQLTransformer (override val uid: String) extends Transformer with Writable\n * class Tokenizer(override val uid: String)\n

[SparkQA]

Test build #46053 has finished for PR 9755 at commit 7e0128c.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class HashingTF(override val uid: String)\n * class Interaction @Since(\"1.6.0\") (override val uid: String) extends Transformer\n * class Normalizer(override val uid: String)\n * class SQLTransformer @Since(\"1.6.0\") (override val uid: String) extends Transformer with Writable\n * class Tokenizer(override val uid: String)\n

[SparkQA]

Test build #46054 has finished for PR 9755 at commit d56fab9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class HashingTF(override val uid: String)\n * class Interaction @Since(\"1.6.0\") (override val uid: String) extends Transformer\n * class Normalizer(override val uid: String)\n * class SQLTransformer @Since(\"1.6.0\") (override val uid: String) extends Transformer with Writable\n * class Tokenizer(override val uid: String)\n

[jkbradley]

Don't commit yet. I just realized I forgot to overload the load method, so it's not Java compatible.

[jkbradley]

OK hopefully good pending tests

[SparkQA]

Test build #46100 has finished for PR 9755 at commit 31b6bc4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class HashingTF(override val uid: String)\n * class Interaction @Since(\"1.6.0\") (override val uid: String) extends Transformer\n * class Normalizer(override val uid: String)\n * class SQLTransformer @Since(\"1.6.0\") (override val uid: String) extends Transformer with Writable\n * class Tokenizer(override val uid: String)\n

[mengxr]

LGTM. Merged into master and branch-1.6. Thanks!