[SPARK-7537] [MLLIB] spark.mllib API updates

Minor updates to the spark.mllib APIs:

1. Add `DeveloperApi` to `PMMLExportable` and add `Experimental` to `toPMML` methods.
2. Mention `RankingMetrics.of` in the `RankingMetrics` constructor.

Author: Xiangrui Meng <meng@databricks.com>

Closes #6280 from mengxr/SPARK-7537 and squashes the following commits:

1bd2583 [Xiangrui Meng] organize imports
94afa7a [Xiangrui Meng] mark all toPMML methods experimental
4c40da1 [Xiangrui Meng] mention the factory method for RankingMetrics for Java users
88c62d0 [Xiangrui Meng] add DeveloperApi to PMMLExportable