rake_task: don't fail the build if a dependency cannot be resolved

Some gems may be manually installed. This also helps working around Gem
version parsing exceptions for gems including 'java' in the version.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
plugin: support extra OSGI services

Killbill will inject more than just the APIs - it will also provide a
logger and the path to the root of the plugin (useful for configuration).

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
http: implement the rack handler

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>