Basic support for Paypal Express in Spree

This commit contains enough to do an express checkout authorization (not purchase) from the cart
page, to save all relevant info, and to be able to view some of it in the admin pages. It is very
much work in progress!

Some admin features won't work (eg captures, viewing transactions), since handling of payments
and gateways needs to be generalised first. Also need to get the orders handled like those for
guest checkouts.

It's also tied to GB/UK details at the moment (this will change), and it needs a modified version
of active merchant which will be released/circulated soon.
typo!
fixed field name
cosmetic tweak to button display
Now pulls the account details from the GatewayOptions system in Spree

NB Spree currently wipes all info except the selected gateway when you edit in the admin section. So be careful! I aim to change this soon.

NB hard-coded to use the UK PP express gateway

See the migration in this extension to see what is created/expected

ps. the earlier account details included in this file were for a sandbox account - sorry!
Significant changes: add missing AM libs, adapt to spree 0.8.4+,
fix admin interface

1. My modifications of AM files are now included and over-ride the
   installed lib
2. The admin interface now works, including being able to review
   paypal payments and do captures
3. The code works cleanly with the new checkout representation.
fixed bug - mentions to old gateway sub-module removed
attach current user to order, to enable order tracking etc
remove the gateway config when running "down"
typo on class test
set state_name for shipping address if nothing is found for the given state
typo
typo
header colours now default to white
only add insurance_offered tag when it's present in options, and fix the details tag name
updated the documentation
generalised the config of the gateway and site-specific options, so the code is no longer
tied to a specific UK site...

(changes are explained in the README)
removed debugging code, and fixed currency conv for :max_amount
add to todo list
made "badly configured gateway" message more informative

also the default tax hook returns a more useful 0.0 rather than an empty
hash...