Add DataFrame.spark.apply

[HyukjinKwon]

This PR adds DataFrame.spark.apply:

import databricks.koalas as ks
ks.range(10).spark.apply(lambda sdf: sdf.selectExpr("id + 1"))

   (id + 1)
0         1
1         2
2         3
3         4
4         5
5         6
6         7
7         8
8         9
9        10

[HyukjinKwon]

I will move relevant test cases into here in a separate PR, e.g.) print_schema is in test_dataframe.py.

[codecov-commenter]

Codecov Report

Merging #1536 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1536   +/-   ##
=======================================
  Coverage   94.14%   94.14%           
=======================================
  Files          37       37           
  Lines        8487     8493    +6     
=======================================
+ Hits         7990     7996    +6     
  Misses        497      497           

Impacted Files	Coverage Δ
databricks/koalas/spark.py	90.09% <100.00%> (+0.62%)	⬆️

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[itholic]

Just my opinion, how about Koalas DataFrame or ks.DataFrame rather just DataFrame because we're describing this functions as "Applies a function that takes and returns a Spark DataFrame" ??
I think maybe It can be confused whether the return type is Spark DataFrame or Koalas DataFrame.

[itholic]

LGTM, otherwise.

[ueshin]

LGTM.

[ueshin]

Thanks! merging.