Refactored extension importing.

We now use importlib which means we no longer support Python 2.6.
Also, this refactor properly imports third party extensions which reside
at the root of PYTHONPATH. Previously, either `markdown.extensions.` or
`mdx_` would be appended to any extension name that did not contain a
dot, which required third party extensions to either be in submodules or
use the old `mdx_` naming convention.

This commit is also in preperation for #336. It will now be much easier to
deprecate (and later remove) support for the old ways of handling extension
names.
Allow named extensions to specify the Class Name

If you were to import the class like this:

    from path.to.module import SomeExtensionClass

Then the named extension would be the string:

    "path.to.module:SomeExtensionClass"

This should simplify loading extensions from the command line or
template filters -- expecially when multiple extensions are defined
in a single python module.

The docs still need updating. I'm waiting to update the docs after
implementing #335 and #336 as that will require a major refactor of
that section of the docs anyway.
Various tweaks to release notes.
Merge branch 'master' of https://github.com/waylan/Python-Markdown