## What changes were proposed in this pull request?

This PR improves the `createDataFrame` method to make it also accept datatype string, then users can convert python RDD to DataFrame easily, for example, `df = rdd.toDF("a: int, b: string")`.
It also supports flat schema so users can convert an RDD of int to DataFrame directly, we will automatically wrap int to row for users.
If schema is given, now we checks if the real data matches the given schema, and throw error if it doesn't.

## How was this patch tested?

new tests in `test.py` and doc test in `types.py`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #11444 from cloud-fan/pyrdd.