client: bulk external charges API support

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
client: fix bug in payAllInvoices

When externalPayment wasn't specified, "null" was passed as the amount
causing an error on the server side.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Add new models and apis for direct payments
Merge remote-tracking branch 'origin/bulk-external-charges'
Merge remote-tracking branch 'origin/updated-payment-plugin-api'

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

Conflicts:
	src/main/java/org/killbill/billing/client/KillBillClient.java
pom.xml: prepare 0.0.9 release

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