Changes include
1. Rename sortDF to arrange
2. Add new aliases `group_by` and `sample_frac`, `summarize`
3. Add more user friendly column addition (mutate), rename
4. Support mean as an alias for avg in Scala and also support n_distinct, n as in dplyr

Using these changes we can pretty much run the examples as described in http://cran.rstudio.com/web/packages/dplyr/vignettes/introduction.html with the same syntax

The only thing missing in SparkR is auto resolving column names when used in an expression i.e. making something like `select(flights, delay)` works in dply but we right now need `select(flights, flights$delay)` or `select(flights, "delay")`. But this is a complicated change and I'll file a new issue for it

cc sun-rui rxin

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #6005 from shivaram/sparkr-df-api and squashes the following commits:

5e0716a [Shivaram Venkataraman] Fix some roxygen bugs
1254953 [Shivaram Venkataraman] Merge branch 'master' of https://github.com/apache/spark into sparkr-df-api
0521149 [Shivaram Venkataraman] Changes to make SparkR DataFrame dplyr friendly. Changes include 1. Rename sortDF to arrange 2. Add new aliases `group_by` and `sample_frac`, `summarize` 3. Add more user friendly column addition (mutate), rename 4. Support mean as an alias for avg in Scala and also support n_distinct, n as in dplyr