Fix#271.Allow explicit return None in function returning None.
Add python2eval.test
Move testAbs2_python2 to python2eval.test
Move testUnicode_python2 to python2eval.test
Move testXrangeAndRange_python2 to python2eval.test
Move testIterator_python2 to python2eval.test
Move testEncodeAndDecode_python2 to python2eval.test
Move testHasKey_python2 to python2eval.test
Move testIntegerDivision_python2 to python2eval.test
Move testFloatDivision_python2 to python2eval.test
Move testAnyStr_python2 to python2eval.test
Move testGenericPatterns_python2 to python2eval.test
Move testGenericMatch_python2 to python2eval.test
Move testFromFuturePrintFunction_python2 to python2eval.test
Move testFromFutureImportUnicodeLiterals_python2 to python2eval.test
Move testFromFutureImportUnicodeLiterals2_python2 to python2eval.test
Move testStrUnicodeCompatibility_python2 to python2eval.test
Move testStrUnicodeCompatibilityInBuiltins_python2 to python2eval.test
Move testTupleAsSubtypeOfSequence_python2 to python2eval.test
Fix testTupleAsSubtypeOfSequence_python2