Make the twanager server server configurable

defaulting to wsgiref.simple_server, replacing the cherrypy
wsgiserver, lowering the weight of the codebase.

The mechanism is as follows:

* twanager server is run
* config is inspected for a `wsgi_server` key
* the value of that key (or the default `tiddlyweb.web.server`) is
  imported and expected to contain a `start_server` function
* that function takes a single argument `config`
* the function starts a server hosting the wsgi app provided by
  the `load_app` function

Request for review from @fnd and @pads

related to issue #55
fix tests for change in twanager server
Merge branch 'master' into wsgiref
remove cherrypy from requirements