Updated all images to correct size as placeholders

I replaced all images with either the correct image or an image as close
as possible to what we need in terms of content and dimensions. This is
mostly for placeholding until we receive the correct images.
sponsors page image placeholders
Fixed some html on speaker info page

Twitter link had a height which didn't fit in properly. Simplified some
of the html and css a bit while I was there.
Updated sponsors on front page to use template

Sponsors on front page are now based on template and include gold,
silver and bronze as per sponsorship package details.
Front page speakers from metadata

Updated the speakers on the front page to be generated from metadata
using the first 4 speakers.
missing sponsor images (placeholders)
made speaker blocks on front page links
added neil blakey-milner bio and image
replaced 2go and praekelt with correct images
Updated sponsors with temporary content based on websites

In case we don't get all the information required from sponsors for
their pages, I've pre-emptively gathered information from their websites
which will be a good base. They may want to convey different messaging
and we can update as required.
Corrected layout and html on sponsor info page

Sponsor info page had same html and layout issue as the speakers page
with the spacing. This simplified the html and made the layout look
better
added stubs for Dietrich Featherston
Remove trailing whitespace