Remove require of workflow/draw to prevent warnings on production systems.

As suggested by @lvanderhoeven I have removed the `require "workflow/draw"` since the gems needed to generate dot graphs are causing trouble on production systems.

From the related issue #72:

> In `lib/workflow/draw.rb` the ruby-graphviz and activesupport gems are required on top
> which means on my production boxes every cronned rake task triggers a flood of emails 
> due to the `$stderr.puts "Could not load ...` message unless I include ruby-graphviz which 
> I don't really want on production boxes.

This commit removes the require, adapts the test and fixes the broken examples of how to use this feature in the README and documentation.