attachments should specify binary

Issue: https://github.com/adlnet/xAPI-Spec/issues/107

* attachment's MUST specify a transfer encoding of binary
* LRS SHOULD assume binary transfer encoding even if it was (incorrectly) not specified

There is no need to allow other transfer encodings as these messages are to be sent over HTTP which supports binary
Merge pull request #122 from bscSCORM/binary

attachments should specify binary