[SPARK-2096][SQL] support dot notation on array of struct

~~The rule is simple: If you want `a.b` work, then `a` must be some level of nested array of struct(level 0 means just a StructType). And the result of `a.b` is same level of nested array of b-type.
An optimization is: the resolve chain looks like `Attribute -> GetItem -> GetField -> GetField ...`, so we could transmit the nested array information between `GetItem` and `GetField` to avoid repeated computation of `innerDataType` and `containsNullList` of that nested array.~~
marmbrus Could you take a look?

to evaluate `a.b`, if `a` is array of struct, then `a.b` means get field `b` on each element of `a`, and return a result of array.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #2405 from cloud-fan/nested-array-dot and squashes the following commits:

08a228a [Wenchen Fan] support dot notation on array of struct