Add in the MarkdownReader class.

Mostly a giant wad of code to allow the current rST templates to play nice with
markdown files. Hopefully, this Markdown class not only reads valid Markdown
and valid Markdown metadata, but converts the limited subset of rST metadata
into valid Markdown metadata too. This is achieved by a combination of regex
parsing and filtering of lines, along the way bailing if it looks like it's
valid Markdown

Note that I've commented out the dependency on python-markdown in global.txt;
this is because I see this as a feature request to a piece of software and
figure that there may be a design decision behind this. I've also done my best
to make this code run _without_ the need for the Markdown renderer, however
obviously I do feel like it'd be best for this to be shipped as a dependency!

Also I like my regex string splitter better than I like the original
"string.split(',')"; credited from stackoverflow. Feel free to use this in
place of your splitter, which won't play nice with things I intentionally want
a comma in!

Signed-off-by: Ben Stott <bgbnbigben@contextualsystems.com>
Add in a database-level "how do we markup this thing?".

This is for two reasons:
    1.  Future extensibility; simply add a database field and we get the choice
        of a new radpress-level renderer

    2.  To allow us to not step on the toes of previous marked-up files! The new
        renderer needs to only edit Markdown files, and for those who already have
        articles written in rST we make sure that these are not presumed to be
        Markdown (even though it's the superior format ;))

Signed-off-by: Ben Stott <bgbnbigben@contextualsystems.com>
Templatetags. Because why not.

Signed-off-by: Ben Stott <bgbnbigben@contextualsystems.com>
Make all forms, models and views that are capable of rendering stuff able to
render Markdown

Should be the final commit required for this; it passes all my own local
testing though I haven't really added proper test cases.

Signed-off-by: Ben Stott <bgbnbigben@contextualsystems.com>
Add in a MD note to the README, plus do some quick cleanup.

Signed-off-by: Ben Stott <bgbnbigben@contextualsystems.com>
add enabled variable to get available readers.
rename DATA as CONTEXT_DATA

and do not add unneeded keys that are not used in templates.
add test for checking context_data of radpress.
radpress settings optimization.
separate renderer modules. not completed.
fix migrations to last changes. markup field max_length changed as 20 chars.
add importlib to install_requires if python version is 2.6 or older.
new option: RADPRESS_DEFAULT_READER
rename rst module as restructuredtext.
define markdown reader name as Markdown.
two important methods are added: get_reader, get_markup_choices