Left sided string, fixing inconsistency with Alethzero(#5)

[o-jasper]

Right-padding instead of left padding.. Looks like the pyethereum tester has a string conversion too, so that'd need a little change too.

Oh, looks like serpent.encode_data and serpent.decode_data need some modification aswel. Although maybe the tester itself should expose some such. I tested it with my own encode and decoder on some of my projects.(1, 2)

Develop branch is old, so i sent it to master.

[o-jasper]

Reading serpent.encode_data, i dont think it does with strings what i thought it did. (For linking issues together: #5 and #16.) Since python seems to deal with 32-byte integers, anyway, it would make sense to interpret strings as strings? You can already input an adress as 0x... number. I am not sure what the 40-byte string input is supposed to do.