Workflowdoc

Added in module doc and what not. Most methods do not have examples included, however, the module level example has a good bit in it. 

Downside: this does not compile right now with sphinx. From what I can tell, which took way to much poking at sphinx internals, is that sphinx cannot handle nested class definitions. I'm not positive though, because the errors zero sense. I've juggled all possibly odd characters, deleted the line it complains about (and surrounding text...) without luck. There is, if you throw ``-vvv`` at ``sphinx-build`` a buried traceback due to an attempt to import ``skbio.core.workflow.Workflow.requires``. Why it is trying that? No clue...