py: Implement support for "except Exception as var" clause.

For this, needed to implement DELETE_NAME bytecode (because var bound
in except clause is automatically deleted at its end).
http://docs.python.org/3/reference/compound_stmts.html#except :
"When an exception has been assigned using as target, it is cleared at
the end of the except clause."
Merge pull request #367 from pfalcon/del-name

py: Implement support for "except Exception as var" clause.