Added simple text printing.

The CustomText object contains an Array of Objects that contain a string
.text and optional Number .offset (from the left). The prototype defines
horizontalSpacing, verticalSpacing, and verticalSpacingBlank. The
letters are printed in horizontal order based on the "textA" - "textZ"
sprites, with "textSpace", "textPeriod", and "textExclamationMark".

Fixes #40.