invoice: proposal for new external charges APIs

Merge all calls to simplify the API. We're losing granularity
for permissions, but in retrospect, I'm not sure we really needed
that level of details.

This allows us also to offer a bulk API.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Initial push for new direct payment APIs.
error_code: add new error code for external charges

For now, make sure we only support one currency (the one on the account).

See discussion https://github.com/killbill/killbill/commit/942e214d49e9c7ed89da76d972ee017d2d3ade58#commitcomment-6045547

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
NEWS update
[maven-release-plugin] prepare release killbill-api-0.8.4
[maven-release-plugin] prepare for next development iteration
Merge remote-tracking branch 'origin/updated-payment-plugin-api'

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

Conflicts:
	pom.xml
.idea: import shared settings

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: allow plugins to expose custom properties

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