Remove balloonpanel dependencies from built and docs.

[f1ames]

Since balloonpanel was moved to CKEditor4 repository as a core plugin it is no longer needed in a built proccess and README/docs. So all its usages/mentions probably should be removed.

The problem I see here is that balloonpanel was moved to major branch of CKEditor4 so it will be available starting from 4.6.0. This task should be somehow synchronized with the release. The other thing is that some users may like to build AC for older (than 4.6.0) versions of CKEditor so we should probably mention in the docs how it should be done since 4.6.0 and before (and keep a branch with older gruntfile for < 4.6.0 building?).