[SPARK-5501][SPARK-5420][SQL] Write support for the data source API

This PR aims to support `INSERT INTO/OVERWRITE TABLE tableName` and `CREATE TABLE tableName AS SELECT` for the data source API (partitioned tables are not supported).

In this PR, I am also adding the support of `IF NOT EXISTS` for our ddl parser. The current semantic of `IF NOT EXISTS` is explained as follows.
* For a `CREATE TEMPORARY TABLE` statement, it does not `IF NOT EXISTS` for now.
* For a `CREATE TABLE` statement (we are creating a metastore table), if there is an existing table having the same name ...
  * when `IF NOT EXISTS` clause is used, we will do nothing.
  * when `IF NOT EXISTS` clause is not used, the user will see an exception saying the table already exists.

TODOs:
- [x] CTAS support
- [x] Programmatic APIs
- [ ] Python API (another PR)
- [x] More unit tests
- [ ] Documents (another PR)

marmbrus liancheng rxin

Author: Yin Huai <yhuai@databricks.com>

Closes #4294 from yhuai/writeSupport and squashes the following commits:

3db1539 [Yin Huai] save does not take overwrite.
1c98881 [Yin Huai] Fix test.
142372a [Yin Huai] Merge remote-tracking branch 'upstream/master' into writeSupport
34e1bfb [Yin Huai] Address comments.
1682ca6 [Yin Huai] Better support for CTAS statements.
e789d64 [Yin Huai] For the Scala API, let users to use tuples to provide options.
0128065 [Yin Huai] Short hand versions of save and load.
66ebd74 [Yin Huai] Formatting.
9203ec2 [Yin Huai] Merge remote-tracking branch 'upstream/master' into writeSupport
e5d29f2 [Yin Huai] Programmatic APIs.
1a719a5 [Yin Huai] CREATE TEMPORARY TABLE with IF NOT EXISTS is not allowed for now.
909924f [Yin Huai] Add saveAsTable for the data source API to DataFrame.
95a7c71 [Yin Huai] Fix bug when handling IF NOT EXISTS clause in a CREATE TEMPORARY TABLE statement.
d37b19c [Yin Huai] Cheng's comments.
fd6758c [Yin Huai] Use BeforeAndAfterAll.
7880891 [Yin Huai] Support CREATE TABLE AS SELECT STATEMENT and the IF NOT EXISTS clause.
cb85b05 [Yin Huai] Initial write support.
2f91354 [Yin Huai] Make INSERT OVERWRITE/INTO statements consistent between HiveQL and SqlParser.