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
first merge from upstream
remove files created
more merge
merge
merge
Fix parsing empty base class lists
Fix failing tests
Add Sphinx docs
Fixed SemanticAnalyzer so it considers all lvalues in assignments to nested tuples.
Recursively type check assignments to nested tupples/lists
Cleanup and small fixes
Remove obsolete file