Yield From Statement and Expression.

Commit's log
=======

First commit
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**nodes.py** ->

    - Added YieldFromStmt

**traverser.py** ->

    - Add import YieldFromStmt
    - Added visit_yield_from function

**visitor.py** ->

    - Added visit_yield_from function

**parse.py** ->

    - Add import YieldFromStmt
    - Modified parse_yield_stmt to allow yield from statements (here comes when there is not assigntment, just a yield from wait)
    - Added parse_yield_from_expr to allow a yield from assigned to a var, return the callExpr of the yield from
    - Modified parse_expresssion, changed the self.current() to t (there is no reason to call it when saved it before) and added a clausule that checks that a assignment expression can go through yield from

Second commit
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**nodes.py** ->

    - Modified YieldFromStmt

**output.py** ->

    - Added visit_yield_from_stmt()

**semanal.py** ->

    - Added YieldFromStmt import
    - Added visit_yield_from_stmt()

**strconv.py** ->

    - Added visit_yield_from_stmt()

**treetransform.py** ->

    - Added YieldFromStmt import
    - Added visit_yield_from_stmt()

**tests/parse-errors.test** ->

    - Add error yield from without right expression

**tests/parse.test** ->

    - Add yield from stmt test
    - Add yield from expression test


Third commit
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**checker.py** ->

    - Add import YieldFromExpr
    - Add visit_yield_from_expr

**checkexpr.py** ->

    - Add import YieldFromExpr
    - Add visit_yield_from_expr

**icode.py** ->

    - Add import YieldFromExpr
    - Add visit_yield_from_expr

**nodes.py** ->

    - Add YieldFromExpr class

**output.py** ->

    - Add visit_yield_from_expr

**parse.py** ->

    - Add import YieldFromExpr
    - Modify return check to fail if is continued by YieldFromExpr
    - Modify visit_yield_from_expr to return a YieldFromExpr instance

**pprinter.py** ->

    - Add visit_yield_from_expr

**semanal.py** ->

    - Add import YieldFromExpr
    - Add visit_yield_from_expr

**stats.py** ->

    - Add import YieldFromExpr
    - Add visit_yield_from_expr

**strconv.py** ->

    - Add visit_yield_from_expr

**transform.py** ->

    - Add import YieldFromExpr
    - Add visit_yield_from_expr

**traverser.py** ->

    - Add import YieldFromExpr
    - Add visit_yield_from_expr

**treetransform.py** ->

    - Add import YieldFromExpr
    - Add visit_yield_from_expr

**visitor.py** ->

    - Add visit_yield_from_expr


**test/data/parse-errors.test** ->


    - Add testYieldFromAfterReturn
**test/data/parse.test** ->

    - Modify testYieldFromExpr with the new representation.


Four commit
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    Merge with master to do a clean pull request