[SPARK-9324][SPARK-9322][SPARK-9321][SPARKR] Some aliases for R-like functions in DataFrames

[falaki]

Adds following aliases:

• unique (distinct)
• rbind (unionAll): accepts many DataFrames
• nrow (count)
• ncol
• dim
• names (columns): along with the replacement function to change names

[SparkQA]

Test build #38941 has finished for PR 7764 at commit d9307f8.

• This patch fails SparkR unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[shivaram]

typo: number of

[shivaram]

Jenkins, retest this please

[SparkQA]

Test build #39123 has finished for PR 7764 at commit 5e4a4d0.

• This patch fails SparkR unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[shivaram]

@falaki the documentation builds are failing with an error message

Error : /home/jenkins/workspace/SparkPullRequestBuilder@2/R/pkg/man/nrow.Rd: Sections \title, and \name must exist and be unique in Rd files

[SparkQA]

Test build #39266 has finished for PR 7764 at commit 56016f5.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[shivaram]

theres an extra line of space before this setMethod, but I can fix this up during merge

[shivaram]

Thanks @falaki for the update. LGTM. I had a minor style fix that I'll apply during the merge.