Use Entity::Mapper for building SOAP data
Promote Entity::Mapper-style SOAP building to Entity
Let handles worry about how to represent themselves in SOAP data
Allow Handle to model currency and VAT handles

Basically this means Entity::Handle now supports :code and :vat_code keys
Handle properties accepts Hash inputs

Even though we're moving towards handles being full-fledged objects, we'd still
like to preserve the existing, easier way of setting handles as Hashes.

With this commit the following two are equivalent:

  Debtor.new.currency_handle = {:code => "USD"}
  Debtor.new.currency_handle = Entity::Handle.new({:code => "USD"})
Remove duplication in defining supported keys for Handles
Remove unnecessary to_i check

It was basically there to ensure we caught all instances of

    :handle => {:id => {:id => 42}}

as we changed the code around to use Handles. It is no longer necessary.

It is also slighty wrong, as we now support keys with String values. While they
do respond to to_i we don't actually call to_i on them.
Clean up Entity a bit
Prepare next release [ci skip]
Entity Handles should be able to support Name keys as used in Account
Use Entity::Mapper
Looks like we might be ready to test on JRuby again

... no more 15+ minutes test runs
Clarify licensing. Closes #16
Split the has_properties in more lines

For CurrentInvoice
It is possible to add text lines to the current invoice