Serpent overhaul: fix string types

[axic]

As discussed with @tinybike, the s type in Serpent actually support arbitrary binary data. Mapping it to string here would cause it to be processed as UTF8 possibly resulting in invalid output.

With using bytes the caller must make sure to encode using UTF8 if needed by supplying the result of new Buffer("hello", 'utf8') as the input.

Fixes #15 #16.

[coveralls]

Coverage decreased (-14.9%) to 66.699% when pulling 8706e93 on patch/serpent-fixes into 0d17bad on master.

[tinybike]

Beautifully done, @axic!