Fix multilevel lists

[Crozzers]

This PR fixes #275. The issue was that lists with multiple levels of nesting were being flattened. For example:

* Item 2
  - Subitem B following:
    + What
    + The
    + Code

Would become:

Which is obviously wrong. The bug came from the following line in the _list_item_sub function:

python-markdown2/lib/markdown2.py

Line 1801 in 6032f8b

item = self._do_lists(self._outdent(item))

Once the above example reaches this line it is then outdented to:

Item 2
- Subitem B following:
+ What
+ The
+ Code

Which now just resembles a normal list and will be processed as such.
This PR changes the _list_item_sub function to use the _uniform_outdent function, which produces the following output at the same point in the program, allowing nesting to be preserved.

Item 2
- Subitem B following:
  + What
  + The
  + Code

Whilst working on this bug I also discovered that the break_on_newline_excessive_br_tags test case, added in PR #422, contained incorrect list nesting.

This PR also fixes that test case.

[nicholasserra]

Doing a quick look at this and it seems wrong because the LIs arent closing. But i'm guessing this is intentional? UL should be nested under the LI before it's closed?

[Crozzers]

The Mozilla docs on nesting lists give an example where the nested <ul> is placed as a child of the <li> before closing and I also found this stackoverflow answer that confirms this

[nicholasserra]

Thank you!