Overhaul style functions

This is a relatively large change to the style machinery. To quote from the release notes:

- The functions that interact with matplotlib rc parameters have been updated and standardized. There are now three pairs of functions, :func:`axes_style` and :func:`set_style`, :func:`plotting_context` and :func:`set_context`, and :func:`color_palette` and :func:`set_palette`. In each case, the pairs take the exact same arguments. The first function defines and returns the parameters, and the second sets the matplotlib defaults. Additionally, the first function in each pair can be used in a ``with`` statement to temporarily change the defaults. Both the style and context functions also now accept a dictionary of matplotlib rc parameters to override the seaborn defaults, and :func:`set` now also takes a dictionary to update any of the matplotlib defaults. See the :ref:`tutorial <aesthetics_tutorial>` for more information.

- The ``nogrid`` style has been deprecated and changed to ``white`` for more uniformity (i.e. there are now ``darkgrid``, ``dark``, ``whitegrid``, and ``white`` styles).

This closes #121, closes #74, and closes #112.

Also @phobson this reverts the addition of the `gridweight` parameter. Because `set()` and `set_style()` accept an `rc` dict with params to override the seaborn themes, it didn't seem necessary to have the additional layer of complexity where some but not all matplotlib parameters are aliased and get special treatment. Sorry if that breaks some code.

This also standardizes the set of parameters that are part of a seaborn style and a seaborn context (and enforces independence between those groups), which should make it easy to integrate with the stylesheet functionality in matplotlib 1.4 (although to be honest I haven't looked too closely at that stuff).

I also took this opportunity to make some tweaks to the styles, with things like thinner gridlines and replacing black elements with dark gray where appropriate.

Because this required rerunning the test notebooks, this PR also includes a downsizing of the aesthetics and linear model notebooks. I'm keeping them around because a lot of links out there on the internet point at the nbviewer versions, but now they're brief introductions with links to the longer version in the online docs.