Fixed built-in authentication

Broke it when removing Authake integration :(
Added support to download datasets as csv, json or xml

It's a little hackey but I did it this way to support downloading
possibly huge data sets without running out of memory (uses about 10MB)
to export around 40k entries.

My local install with a limit of 128MB runs out of memory if we process
this the normal way (build an array of 40k results) and generate the
view. And I didn't want to impose a record limit for exports.

If someone has a better approach, by all means fix it :).
Misc cosmetic fixes, fixed pagination with filters active

Fixed pagination links if you are filtering (before it was just paging
on the normal data set). You can now page over your filtered data set
:D.

Cosmetic fix for Baked Actions links, it's now a little scrollable link
in the header area. This could be better though

Hid the default cakephp pagination links (display:none;)
Tweak to only show download links on admin_index pages