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 .travis.yml
Add build status to README
Remove logic for running a file after translation

Bug #373
Add a arrow symbol (->) to the lexer
Add a arrow symbol (->) to the lexer

Bug #368
Add a test for the arrow operator

Bug #386
Rename test function
When parsing type, expect a single '->' token
Make sure test failures are tracked
PEP 8 fixes for build.py
PEP 8 fixes for cgen.py
PEP 8 fixes for checker.py