Add Html unit tests for adding/removing servers.

Add Html unit tests for triggering from multiple Gerrit servers.

Change-Id: I6fe2be3a1091c88f30a4dc6b470f2e4abf8873fe
Merge pull request #94 from anthonyChin/htmlunit-tests

Add Html unit tests for adding/removing servers.
Fix JENKINS-20098

Also bumping core dependency to 1.480
it would be strange to keep to 1.424
since git-client requires 1.480.
Merge pull request #93 from slawekjaranowski/master

Added missing copyright header.
NPE Fix
Merge branch 'master' of github.com:jenkinsci/gerrit-trigger-plugin