Add expense creation service

This adds a route at POST /expense that accepts an expense
representation and creates it in the database against the current signed
in user.

A new database table `expense` has been created along with a companion
Clojure namespace at `expensapp.expenses` containing the queries that
will be used.
Update User table details in README
Clean up README formatting
Store money as a numeric in postgres

The numeric column definition sets a precision of six and a scale of
two, meaning that the number can have anything from the range of
-9999.99 up to 9999.99.

The numbers are completely arbitrary. I don't think we'll be needing
anything wider ranging.