All processors and patterns now get "registered" to a Registry.
Each item is given a name (string) and a priority. The name is for
later reference and the priority can be either an integer or float
and is used to sort. A Registry instance is a list-like iterable with
the items auto-sorted by priority. If two items have the same priority,
then they are listed in the order there were "registered". Registering
a new item with the same name as an already registered item replaces
the old item with the new item (however, the new item is sorted by
its priority). To remove an item, "deregister" it by name or index.

Fixes #418.

Previously, instances of the Markdown class were represented as any one of
'md', 'md_instance', or 'markdown'. This inconsistancy made it diffcult
when developing extensions, or just maintaining the existing code.
Now, all instances are consistantly represented as 'md'.

Got all but a couple files in the tests (ran out of time today).
Apparently I have been using some bad form for years (although a few
things seemed to look better before the update). Anyway, conformant now.

A couple scenarios with "None" were previously not accounted for.
Also updated tests which guives us 100% for markdown/util.py

Fixes #325. All extensions can now accept a dict of configs or
**kwargs, not just a list of tuples. Third party extensions may want
to follow suite. Extensions may only accept keyword arguments
in the future. These changes still need to be documented.

A couple things of note:

The CodeHilite extension previously issued a DeprecationWarning
if the old config key `force_linenos` was used. With thins change,
a KeyError will now be raised.

The `markdown.util.parseBoolValue` function gained a new argument:
`preserve_none` (defaults to False), which when set to True, will
pass None through unaltered (will not convert it to False).

This was limiting the parser to only 10,000 placeholders (0-9999) per document.
removing this limitation allows longer documents ot be parsered correctly.
Fixes #255.