[SPARK-9463] [ML] Expose model coefficients with names in SparkR RFormula

Preview:

```
> summary(m)
            features coefficients
1        (Intercept)    1.6765001
2       Sepal_Length    0.3498801
3 Species.versicolor   -0.9833885
4  Species.virginica   -1.0075104

```

Design doc from umbrella task: https://docs.google.com/document/d/10NZNSEurN2EdWM31uFYsgayIPfCFHiuIu3pCWrUmP_c/edit

cc mengxr

Author: Eric Liang <ekl@databricks.com>

Closes #7771 from ericl/summary and squashes the following commits:

ccd54c3 [Eric Liang] second pass
a5ca93b [Eric Liang] comments
2772111 [Eric Liang] clean up
70483ef [Eric Liang] fix test
7c247d4 [Eric Liang] Merge branch 'master' into summary
3c55024 [Eric Liang] working
8c539aa [Eric Liang] first pass