Improve error message when the type is not bytes

[zkanda]

Fixes #5
This work on both Python 2 and 3 but I'm not sure what a proper test for it.

A simple test like this would work in py3 but fails in py2 because data is both str and bytes:

def test_input_should_be_bytes():
     data = '3vQB7B6MrGQZaxCuFg4oH'
     with assert_raises(TypeError):
         b58encode(data)

[keis]

Thanks @zkanda

Added that tests but with a unicode string which should properly fail on both 2 and 3.