Add UploadedFile base class and Spreadsheet impl

We need protection around uploaded spreadsheet that are being submitted
for addition to a data set.

This needs to check whether the file looks right:
  - Is not too big or too small,
  - Has a valid content-type,
  - Not a virus!

I've pulled in the csv and excel parsing code from Backdrop and tweaked
them to fit the new admin app. This was done to retain a certain amount
of parity between the new and old uploader.
Try and make Excel do integers

All the numbers that come out of excel spreadsheets are floating point,
but we wan't to be able to define columns as being integers. This does a
little python fudge to see if the number is a valid integer and then
converts it.
Add a script to run the app

This is used by the Procfile in pp-puppet
Merge pull request #14 from alphagov/add-start-app

Add a script to run the app
Merge pull request #12 from alphagov/upload-spreadsheets

Upload spreadsheets
Add in assets and compilation from govuk_admin_template and bootstrap 3.1.0

compilation uses pyscss by way of flask scss
it should auto compile on start and first page load
Add manual scss compilation script for deploy job

uses pyScss
Add readme explaining auto compilation in dev and manual compiling
Add placeholder routes for testing templates
Better index page

This is somewhat in line with henry's designs but I haven't strayed too
far from what the admin template styles do by default
There are also some decisions to be made about whether this is ever seen
after sign in and some Jinja logic to fill in
Add data set list and upload form partial

Jinja logic needs filling in
Again, inline with designs, not close to pixel perfect
Upload error template

does not include table
Upload okay partial for use with the data sets list
Add in jinja logic