Support DT.BIN field length data type during parsing

[rjarce]

Hi!

I did some changes to further support parsing of binary-encoded ISO 8583 messages. Currently, when parsing variable length fields (LLVAR or LLLVAR), DT.BIN length types are ignored, so the actual length of the field is not getting parsed.

BTW, I pulled @aguirrel's changes into my own fork as I'm encountering an error when importing py8583spec from a test project. Can you tell me how to import the library from other projects using the current master implementation?

Thanks!