add missing sjcl.codec.bytes

still tons of tests failing, but at least not from this
The byte codec is already a part of sjcl, it's just not part of their default build.
It is already included in [sjcl.config.mk](https://github.com/stellar/stellar-lib/blob/master/config/sjcl.config.mk), which is used in [Gulpfile.js](https://github.com/stellar/stellar-lib/blob/master/Gulpfile.js#L54) to customize our build of sjcl.

Can you run ```gulp``` and make sure the "sjcl" task is running successfully?