[SPARK-11339][SPARKR] Document the list of functions in R base package that...

[SPARK-11339][SPARKR] Document the list of functions in R base package that are masked by functions with same name in SparkR

Added tests for function that are reported as masked, to make sure the base:: or stats:: function can be called.

For those we can't call, added them to SparkR programming guide.

It would seem to me `table, sample, subset, filter, cov` not working are not actually expected - I investigated/experimented with them but couldn't get them to work. It looks like as they are defined in base or stats they are missing the S3 generic, eg.
```
> methods("transform")
[1] transform,ANY-method       transform.data.frame
[3] transform,DataFrame-method transform.default
see '?methods' for accessing help and source code
> methods("subset")
[1] subset.data.frame       subset,DataFrame-method subset.default
[4] subset.matrix
see '?methods' for accessing help and source code
Warning message:
In .S3methods(generic.function, class, parent.frame()) :
  function 'subset' appears not to be S3 generic; found functions that look like S3 methods
```
Any idea?

More information on masking:
http://www.ats.ucla.edu/stat/r/faq/referencing_objects.htm
http://www.sfu.ca/~sweldon/howTo/guide4.pdf

This is what the output doc looks like (minus css):
![image](https://cloud.githubusercontent.com/assets/8969467/11229714/2946e5de-8d4d-11e5-94b0-dda9696b6fdd.png)

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #9785 from felixcheung/rmasked.