All extension tests now use python dot notation.

All named extensions now use python dot notation in the tests - including
all builtin extensions (eg: 'extra' => 'markdown.extensions.extra'). This
is in anticipation of #336.

Note there are a few tests (in the error tests) that will still need
updating, but not till we make the change as they will test for the
new error message.
Docs now use dot notation for all extensions.

Except were "short names" are explained in the docs, all references
to the buitlin extensions now use `markdown.extensions.*` in
anticipation of #336.
More updates to test configs.

The last few extensions were updated to accept dicts/**kwargs as configs and more
tests were updated. Also updated extra to actually accept configs.

Note that extra requires an extra level of dicts. First you need
to indicate tha the settings are for extra, then, which extension
extra wraps. I'm not crazy abount this, bit not sur ehow else to do it
without making all the configs a global attribute on the Markdown class
to that any extention can access any other extensions config settings.
I don't think we wnat to do that.

Also updated extra to use dot notation for the sub-extensions.
Refactored tests to no longer pass configs to names extensions.

`'path.to.module(option=value'` => `path.to.module.Class(option='value')`
This is in anticipation of #335
Code exampeles in extension docs now show best practices.

This is in anticipation of #335. The reference and extension api docs still
need to be updated, but that will happen with change in the code.