fixed cp35212 - allow mixing string and bytes in args of string methods

all string methods accept bytes in place of string
all string methods accept extensible string in place of string
all bytes methods accept strings in place of bytes
corrected argument names for replace
added dedicated test_str_bytes.py
simplified mixing of buffer as argument of str.replace
added buffer and bytearray to types covered by BytesConversion attribute
corrected replace parameter names
removed no longer used function
Merge pull request #198 from paweljasinski/cp35212

 fixed cp35212 - allow mixing string and bytes in args of string methods
corrected parameter names of str.replace