Add module_size parameter to encode function

[sirapu]

Hi,

I added the parameter for defining the modules size to the encode function. Default seems to be 5 pixels.

Taken from manual page of the dmtxwrite command:
-d, --module=N
Module size in pixels.

regards,
Alexander

[coveralls]

Coverage decreased (-0.09%) to 96.329% when pulling 1816593 on sirapu:feature/50-encode-module-parameter into a425ec3 on NaturalHistoryMuseum:master.