Yield from statements and expressions support.
Files changed:
nodes.py ->
    - Added YieldFromStmt, extends from YieldStmt
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

TODO:
tests (I made "by hand" but it is not automated)
First steps to the yield from expr
Add YieldFromExpr class and the visitors. Change return to check if it's not yield from
Add YieldFromExpr class and the visitors. Change return to check if it's not yield from
Errors Correct
========

"yield lambda" error correct
-------

**parse.py** ->

    - Modify visit_yield: Check if the next token is "from" before to "expect" it.

"Can't yield from a NameExpr" error correct
--------

**checkexpr.py** ->

    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.

**icode.py** ->

    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.

**output.py** ->

    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.

**pprinter.py** ->

    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.

**semanal.py** ->

    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.

**stats.py** ->

    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.

**strconv.py** ->

    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.

**transform.py** ->

    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.

**traverser.py** ->
    - Add NameExpr to import
    - Check if expression of yield from is a CallFunc or a NameExpr and visit the correct one.
Yield from stmt and expr wit type checks
Merge to do a clean pull_request
Add fail if the difference of Futures are 0
Added parse rule for comparison expressions
Added type checking of comparison expression
Adapted tests for new ComparisonExpr node
Added proper evaluation of resulting type of comparison expressions instead of always returning bool
Fixed DyncheckTransformVisitor for comparison expressions and related bugs.
Removed remaining code associated with OpExpr for 'in' and 'is' operators, which were are handled by ComparisonExpr
Added tests for chain comparison expresions
Removed transform code for comparison expresions
Fix #429.Refactor overloads away in 'random' stubs
Merge pull request #440 from yczhu/refactor-overloads

Fix #429.Refactor overloads away in 'random' stubs
Fix #433. Refactor overloads away in zipfile stubs
Made code parsable by mypy