[SPARK-4574][SQL] Adding support for defining schema in foreign DDL commands.

Adding support for defining schema in foreign DDL commands. Now foreign DDL support commands like:
```
CREATE TEMPORARY TABLE avroTable
USING org.apache.spark.sql.avro
OPTIONS (path "../hive/src/test/resources/data/files/episodes.avro")
```
With this PR user can define schema instead of infer from file, so  support ddl command as follows:
```
CREATE TEMPORARY TABLE avroTable(a int, b string)
USING org.apache.spark.sql.avro
OPTIONS (path "../hive/src/test/resources/data/files/episodes.avro")
```

Author: scwf <wangfei1@huawei.com>
Author: Yin Huai <yhuai@databricks.com>
Author: Fei Wang <wangfei1@huawei.com>
Author: wangfei <wangfei1@huawei.com>

Closes #3431 from scwf/ddl and squashes the following commits:

7e79ce5 [Fei Wang] Merge pull request #22 from yhuai/pr3431yin
38f634e [Yin Huai] Remove Option from createRelation.
65e9c73 [Yin Huai] Revert all changes since applying a given schema has not been testd.
a852b10 [scwf] remove cleanIdentifier
f336a16 [Fei Wang] Merge pull request #21 from yhuai/pr3431yin
baf79b5 [Yin Huai] Test special characters quoted by backticks.
50a03b0 [Yin Huai] Use JsonRDD.nullTypeToStringType to convert NullType to StringType.
1eeb769 [Fei Wang] Merge pull request #20 from yhuai/pr3431yin
f5c22b0 [Yin Huai] Refactor code and update test cases.
f1cffe4 [Yin Huai] Revert "minor refactory"
b621c8f [scwf] minor refactory
d02547f [scwf] fix HiveCompatibilitySuite test failure
8dfbf7a [scwf] more tests for complex data type
ddab984 [Fei Wang] Merge pull request #19 from yhuai/pr3431yin
91ad91b [Yin Huai] Parse data types in DDLParser.
cf982d2 [scwf] fixed test failure
445b57b [scwf] address comments
02a662c [scwf] style issue
44eb70c [scwf] fix decimal parser issue
83b6fc3 [scwf] minor fix
9bf12f8 [wangfei] adding test case
7787ec7 [wangfei] added SchemaRelationProvider
0ba70df [wangfei] draft version