Remove instance field in Plugin class

Jenkins creates only one instance from Plugin class then manage it.
So the field which keeps own instance is not needed.

Instead, we should use getPlugin(Class<T> clazz) in Jenkins class.
Change the implementation of server list to CopyOnWriteList

The purpose of this change is to throw away some synchronized methods.

Plugin instance is accessed from a lot of threads. So synchronized
methods block many processes. It would become a critical issue
if implement remote API.

This change solves such situation by using CopyOnWriteList.
It will improve UI performance.
Add remote API

This change adds remote API support.

You can get the status of Gerrit server using belowi url:

$JENKINS_URL/plugin/gerrit-trigger/api/
Fix checkstyle errors
Merge pull request #186 from rinrinne/export-api

Add support for remote API