A TiddlyWeb plugin which adds a _hash field to outgoing tiddlers.

* When a tiddler is get from the store, if there is no _hash, then the
  hash is added to the outgoing tiddler. It is _not_ saved

This is done to allow quick comparisons between tiddlers which may have
different names but may have the same set of content characteristics.

The hash_tiddler method is exposed so that stores can create a hash
prior to a put to the store. See tiddlywebplugis.tiddlyspace.store for
an example.

To use add 'tiddlywebplugins.hashmaker' to system_plugins and
twanager_plugins in tiddlywebconfig.py. Optionally define 
hashmaker.attributes to be a list of strings representing the attributes
or fields of a tiddler which are to be hashed. The default is:

    ['text']