I added lots of expression functions for SparkR. This PR includes only functions whose params  are only `(Column)` or `(Column, Column)`.  And I think we need to improve how to test those functions. However, it would be better to work on another issue.

## Diff Summary

- Add lots of functions in `functions.R` and their generic in `generic.R`
- Add aliases for `ceiling` and `sign`
- Move expression functions from `column.R` to `functions.R`
- Modify `rdname` from `column` to `functions`

I haven't supported `not` function, because the name has a collesion with `testthat` package. I didn't think of the way  to define it.

## New Supported Functions

```
approxCountDistinct
ascii
base64
bin
bitwiseNOT
ceil (alias: ceiling)
crc32
dayofmonth
dayofyear
explode
factorial
hex
hour
initcap
isNaN
last_day
length
log2
ltrim
md5
minute
month
negate
quarter
reverse
round
rtrim
second
sha1
signum (alias: sign)
size
soundex
to_date
trim
unbase64
unhex
weekofyear
year

datediff
levenshtein
months_between
nanvl
pmod
```

## JIRA
[[SPARK-9855] Add expression functions into SparkR whose params are simple - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-9855)

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #8123 from yu-iskw/SPARK-9855.