[SPARK-2053][SQL] Add Catalyst expressions for CASE WHEN.

JIRA ticket: https://issues.apache.org/jira/browse/SPARK-2053

This PR adds support for two types of CASE statements present in Hive. The first type is of the form `CASE WHEN a THEN b [WHEN c THEN d]* [ELSE e] END`, with the semantics like a chain of if statements. The second type is of the form `CASE a WHEN b THEN c [WHEN d THEN e]* [ELSE f] END`, with the semantics like a switch statement on key `a`. Both forms are implemented in `CaseWhen`.

[This link](https://cwiki.apache.org/confluence/display/Hive/LanguageManual+UDF#LanguageManualUDF-ConditionalFunctions) contains more detailed descriptions on their semantics.

Notes / Open issues:

* Please check if any implicit contracts / invariants are broken in the implementations (especially for the operators). I am not very familiar with them and I currently find them tricky to spot.
* We should decide whether or not a non-boolean condition is allowed in a branch of `CaseWhen`. Hive throws a `SemanticException` for this situation and I think it'd be good to mimic it -- the question is where in the whole Spark SQL pipeline should we signal an exception for such a query.

Author: Zongheng Yang <zongheng.y@gmail.com>

Closes #1055 from concretevitamin/caseWhen and squashes the following commits:

4226eb9 [Zongheng Yang] Comment.
79d26fc [Zongheng Yang] Merge branch 'master' into caseWhen
caf9383 [Zongheng Yang] Update a FIXME.
9d26ab8 [Zongheng Yang] Add @transient marker.
788a0d9 [Zongheng Yang] Implement CastNulls, which fixes udf_case and udf_when.
7ef284f [Zongheng Yang] Refactors: remove redundant passes, improve toString, mark transient.
f47ae7b [Zongheng Yang] Modify queries in tests to have shorter golden files.
1c1fbfc [Zongheng Yang] Cleanups per review comments.
7d2b7e2 [Zongheng Yang] Translate CaseKeyWhen to CaseWhen at parsing time.
47d406a [Zongheng Yang] Do toArray once and lazily outside of eval().
bb3d109 [Zongheng Yang] Update scaladoc of a method.
aea3195 [Zongheng Yang] Fix bug that branchesArr is not used; remove unused import.
96870a8 [Zongheng Yang] Turn off scalastyle for some comments.
7392f3a [Zongheng Yang] Minor cleanup.
2cf08bb [Zongheng Yang] Merge branch 'master' into caseWhen
9f84b40 [Zongheng Yang] Add golden outputs from Hive.
db51a85 [Zongheng Yang] Add allCondBooleans check; uncomment tests.
3f9ef0a [Zongheng Yang] Cleanups and bug fixes (mainly in eval() and resolved).
be54bc8 [Zongheng Yang] Rewrite eval() to a low-level implementation. Separate two CASE stmts.
f2bcb9d [Zongheng Yang] WIP
5906f75 [Zongheng Yang] WIP
efd019b [Zongheng Yang] eval() and toString() bug fixes.
7d81e95 [Zongheng Yang] Clean up resolved.
a31d782 [Zongheng Yang] Finish up Case.