Async tokenizer, PdfColor wrapper, Font subsetting

This release add's a handful of minor api changes including async content tokenizer reader, a new PdfColor class, Document.append now accepts an array of documents, and Font subsetting. This release also includes the initial push towards unit testing using alsatian. This move is to help expand the breadth of tests for nopodofo by utilizing the many features alsatian offers.
Documentation has also been moved from docs generated with typedoc to docs that will be maintained and created by myself and hopefully other maintainers one day. Documentation is now provided as a gitbook and accessible via 'the book' from the README.