SPARK-2096 Correctly parse dot notations

First let me write down the current `projections` grammar of spark sql:

    expression                : orExpression
    orExpression              : andExpression {"or" andExpression}
    andExpression             : comparisonExpression {"and" comparisonExpression}
    comparisonExpression      : termExpression | termExpression "=" termExpression | termExpression ">" termExpression | ...
    termExpression            : productExpression {"+"|"-" productExpression}
    productExpression         : baseExpression {"*"|"/"|"%" baseExpression}
    baseExpression            : expression "[" expression "]" | "(" expression ")" | ident | ...
    ident                     : identChar {identChar | digit} | delimiters | ...
    identChar                 : letter | "_" | "."
    delimiters                : "," | ";" | "(" | ")" | "[" | "]" | ...
    projection                : expression [["AS"] ident]
    projections               : projection { "," projection}

For something like `a.b.c[1]`, it will be parsed as:
<img src="http://img51.imgspice.com/i/03008/4iltjsnqgmtt_t.jpg" border=0>
But for something like `a[1].b`, the current grammar can't parse it correctly.
A simple solution is written in `ParquetQuerySuite#NestedSqlParser`, changed grammars are:

    identChar                 : letter | "_"
    baseExpression            : expression "[" expression "]" | expression "." ident | "(" expression ")" | ident | ...
This works well, but can't cover some corner case like `select t.a.b from table as t`:
<img src="http://img51.imgspice.com/i/03008/v2iau3hoxoxg_t.jpg" border=0>
`t.a.b` parsed as `GetField(GetField(UnResolved("t"), "a"), "b")` instead of `GetField(UnResolved("t.a"), "b")` using this new grammar.
However, we can't resolve `t` as it's not a filed, but the whole table.(if we could do this, then `select t from table as t` is legal, which is unexpected)
My solution is:

    dotExpressionHeader       : ident "." ident
    baseExpression            : expression "[" expression "]" | dotExpressionHeader | expression "." ident | "(" expression ")" | ident | ...
I passed all test cases under sql locally and add a more complex case.
I'm not familiar with the latter optimize phase, please correct me if I missed something.