Second part of the fix for yield inside a try-finally

As noted in #600, Python 3 allows a return inside a generator
method, that raises a StopIteration and passes the return value
inside the 'value' attr of the exception.

To allow this behaviour we simple set 'contains_yield' while compiling
'yield', thus allowing a return statement, but only for Python 3. Then
when compiling the try-except, we check for contains_yield to decide
whether there will be a return.

This allows code like:

(defn gen []
  (yield 3)
  "goodbye")

to compile in both Py2 and Py3. The return value is simply ignored in
Python 2.

hy2py in Python 2 gives:

def g():
    yield 3L
    u'goodbye'

while hy2py in Python 3 gives:

def g():
    yield 3
    return 'goodbye'