[SPARK-5586][Spark Shell][SQL] Make `sqlContext` available in spark shell

Result is like this
```
15/02/05 13:41:22 INFO SparkILoop: Created spark context..
Spark context available as sc.
15/02/05 13:41:22 INFO SparkILoop: Created sql context..
SQLContext available as sqlContext.

scala> sq
sql          sqlContext   sqlParser    sqrt
```

Author: OopsOutOfMemory <victorshengli@126.com>

Closes #4387 from OopsOutOfMemory/sqlContextInShell and squashes the following commits:

c7f5203 [OopsOutOfMemory] auto-import sql() function
e160697 [OopsOutOfMemory] Merge branch 'sqlContextInShell' of https://github.com/OopsOutOfMemory/spark into sqlContextInShell
37c0a16 [OopsOutOfMemory] auto detect hive support
a9c59d9 [OopsOutOfMemory] rename and reduce range of imports
6b9e309 [OopsOutOfMemory] Merge branch 'master' into sqlContextInShell
cae652f [OopsOutOfMemory] make sqlContext available in spark shell