SPARK-2186: Spark SQL DSL support for simple aggregations such as SUM and AVG

**Description** This patch enables using the `.select()` function in SchemaRDD with functions such as `Sum`, `Count` and other.
**Testing** Unit tests added.

Author: Ximo Guanter Gonzalbez <ximo@tid.es>

Closes #1211 from edrevo/add-expression-support-in-select and squashes the following commits:

fe4a1e1 [Ximo Guanter Gonzalbez] Extend SQL DSL to functions
e1d344a [Ximo Guanter Gonzalbez] SPARK-2186: Spark SQL DSL support for simple aggregations such as SUM and AVG
[SPARK-2328] [SQL] Add execution of `SHOW TABLES` before `TestHive.reset()`.

`PruningSuite` is executed first of Hive tests unfortunately, `TestHive.reset()` breaks the test environment.
To prevent this, we must run a query before calling reset the first time.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1268 from ueshin/issues/SPARK-2328 and squashes the following commits:

043ceac [Takuya UESHIN] Add execution of `SHOW TABLES` before `TestHive.reset()`.
[SPARK-2287] [SQL] Make ScalaReflection be able to handle Generic case classes.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1226 from ueshin/issues/SPARK-2287 and squashes the following commits:

32ef7c3 [Takuya UESHIN] Add execution of `SHOW TABLES` before `TestHive.reset()`.
541dc8d [Takuya UESHIN] Merge branch 'master' into issues/SPARK-2287
fac5fae [Takuya UESHIN] Remove unnecessary method receiver.
d306e60 [Takuya UESHIN] Merge branch 'master' into issues/SPARK-2287
7de5706 [Takuya UESHIN] Make ScalaReflection be able to handle Generic case classes.
[SPARK-2324] SparkContext should not exit directly when spark.local.dir is a list of multiple paths and one of them has error

The spark.local.dir is configured as a list of multiple paths as follows /data1/sparkenv/local,/data2/sparkenv/local. If the disk data2 of the driver node has error, the application will exit since DiskBlockManager exits directly at createLocalDirs. If the disk data2 of the worker node has error, the executor will exit either.
DiskBlockManager should not exit directly at createLocalDirs if one of spark.local.dir has error. Since spark.local.dir has multiple paths, a problem should not affect the overall situation.
I think DiskBlockManager could ignore the bad directory at createLocalDirs.

Author: yantangzhai <tyz0303@163.com>

Closes #1274 from YanTangZhai/SPARK-2324 and squashes the following commits:

609bf48 [yantangzhai] [SPARK-2324] SparkContext should not exit directly when spark.local.dir is a list of multiple paths and one of them has error
df08673 [yantangzhai] [SPARK-2324] SparkContext should not exit directly when spark.local.dir is a list of multiple paths and one of them has error
[SPARK] Fix NPE for ExternalAppendOnlyMap

It did not handle null keys very gracefully before.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1288 from andrewor14/fix-external and squashes the following commits:

312b8d8 [Andrew Or] Abstract key hash code
ed5adf9 [Andrew Or] Fix NPE for ExternalAppendOnlyMap
SPARK-1675. Make clear whether computePrincipalComponents requires centered data

Just closing out this small JIRA, resolving with a comment change.

Author: Sean Owen <sowen@cloudera.com>

Closes #1171 from srowen/SPARK-1675 and squashes the following commits:

45ee9b7 [Sean Owen] Add simple note that data need not be centered for computePrincipalComponents
[SPARK-2342] Evaluation helper's output type doesn't conform to input ty...

The function cast doesn't conform to the intention of "Those expressions are supposed to be in the same data type, and also the return type." comment

Author: Yijie Shen <henry.yijieshen@gmail.com>

Closes #1283 from yijieshen/master and squashes the following commits:

c7aaa4b [Yijie Shen] [SPARK-2342] Evaluation helper's output type doesn't conform to input type