[SPARK-1874][MLLIB] Clean up MLlib sample data

1. Added synthetic datasets for `MovieLensALS`, `LinearRegression`, `BinaryClassification`.
2. Embedded instructions in the help message of those example apps.

Per discussion with Matei on the JIRA page, new example data is under `data/mllib`.

Author: Xiangrui Meng <meng@databricks.com>

Closes #833 from mengxr/mllib-sample-data and squashes the following commits:

59f0a18 [Xiangrui Meng] add sample binary classification data
3c2f92f [Xiangrui Meng] add linear regression data
050f1ca [Xiangrui Meng] add a sample dataset for MovieLensALS example