payment: first pass at extending APIs

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
.idea: import shared settings

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: hosted payment pages APIs iteration

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
NEWS update
[maven-release-plugin] prepare release killbill-plugin-api-0.6.5
[maven-release-plugin] prepare for next development iteration
.idea: update copyright header

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
pom.xml: update to 0.7.0-SNAPSHOT

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: allow custom properties to be passed from/to the plugins

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