that f# api project doesn't really work, gonna do the web bit in c# and f# for the rest
Remove Payment type, use PaymentValue instead

At the moment I don't see the need for a type that is just a payment value
with an Id. If I refactor it in future a payment type that includes notes
might be useful but those can also be added on later and I don't even
think that will be useful.

Basically I think it is better for the basic unit of payments to just be
the value, and not deal with any ids.
Add data-access project

Adds the data access layer, which is called Vault in keeping with the
financial theme. At the moment all it does is get purchases out, and there
are no database files yet because I'm awful and at this stage of the
project I'm not bothered about being unable to immediatly run it after a
git clone.
Add some DB methods

Loading *and* saving purchases should now in theory work, assuming that
since it compiles it will run.