account: payment: add search APIs

This patch implements:

  GET /1.0/kb/accounts/search/search_key
  GET /1.0/kb/paymentMethods/search/search_key

This required an update of killbill-api to 0.4.0 and
killbill-plugin-api to 0.3.0.

This also:

* fixes some jaxrs models which were missing an equals() implementation
* fixes the getPaymentMethodWithPluginInfo implementation in KillbillClient

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>