server: first pass at implementing RBAC

Design document can be found on the wiki:

  https://github.com/killbill/killbill/wiki/Role-Based-Access-Control-(RBAC)

This first patch separates Multi-Tenancy from RBAC: moving forward, the Shiro filter
(and shiro.ini) will only be used for pure RBAC, regardless of Multi-Tenancy (we still
use internally various cryptography bits from Shiro for multi-tenancy authentication).

The main change in the server tests is that Multi-Tenancy is now enabled (the suite runs in its
own tenant). Since all calls go through the authentication flow, tests are a bit slower: between 300ms
to 500ms per call, resulting in a suite running a few minutes longer. On the upside, this found a
few bugs around Multi-Tenancy handling.

Among changes:

* fix a bug in DefaultAccountUserApi where the tenant_record_id wasn't populated on creation
* fix a bug in invoice and payment where the tenantId would be lost
* introduce X-Killbill-ApiKey and X-Killbill-ApiSecret headers for Multi-Tenancy authentication
* turn Multi-Tenancy on by default
* enable TestTenantFilter (no dependency on shiro.ini anymore)
* server tests timeouts have been increased (to work around slowness when priming Shiro)
* .idea: add misc. words to the dictionary

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