Update adler checksum to not use bytebuffer for checksums

[truncs]

Using ByteBuffer for checksums is a java8 feature and we should avoid it.

http://docs.oracle.com/javase/7/docs/api/java/util/zip/Checksum.html

[willsalz]

ok…we'll have to read the ByteBuf to a byte[], run the checksum, then repackage the ByteBuf via Unpooled.wrappedBuffer(<byte[] goes here>. Not a huge deal.

[willsalz]

Done in #36 .