Add supporting classes for manual barcode creations

[Caffe1neAdd1ct]

It's possible but tricky to create your own 2D datamatrix barcode and eye readable 2d identifiers. This is required for very custom Royal Mail integrations where the client will be producing labels on their own invoices / dispatch paper.

Add classes to generate these with as much documentation as possible.

See php-dmtx for barcode generation and LuhnMod16\LuhnMod16 for the checksum implementation.

Add variable stream wrapper for GD to variable png generation without writing to the hard drive, see https://gist.github.com/Caffe1neAdd1ct/2819a72e1b77abf6b4968ffd52c2802e