rlp zero coding

def int_to_big_endian(integer):
    '''convert a integer to big endian binary string'''
    if integer == 0:
        return '' 
    s = '%x' % integer
    if len(s) & 1:
        s = '0' + s
    return s.decode('hex')

def big_endian_to_int(string):
    '''convert a big endian binary string to integer'''
    string = string or '\x00' # cpp client encodes \x00 as ''
    s = string.encode('hex')
    return int(s, 16)