client: change internal representation of query params

Use Multimap instead of Map, to support multiple query
parameters with the same name (e.g. pluginProperty).

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
.idea: import Kill Bill configuration

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

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

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
client: fix typo in payments search call

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
client: implement more direct payment APIs

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