Fix gkmngrgn/radpress#12.

Creates a new templatetag that takes the current context (which by virtue of
TEMPLATE_CONTEXT_PROCESSORS contains the current request object (plus all the
defaults -- I think this is a stupid system but I don't know that django allows
me to use += ...)) and builds the correct absolute URI for the given path. This
is then placed straight into the template and not attempted to be
parsed/corrected by the javascript code.

Signed-off-by: Ben Stott <bgbnbigben@contextualsystems.com>
Merge pull request #13 from draftable/js_fix

Fix gkmngrgn/radpress#12.
update document. radpress needs a context processor.
add image gallery link to zen mode screen.
initial variable for readers.
DEFAULT_READER -> DEFAULT_MARKUP
get_reader argument name changes as `markup`.
show markdown usage in demo.
Add in the markdown_reader initial content.

Might not be worth keeping the RST-to-MD meta conversion, but eh, I'll leave it
in

Signed-off-by: Ben Stott <bgbnbigben@contextualsystems.com>