IndexJobManager does no longer inherit from Index

There is no need for ``IndexJobManager`` to inherit from ``Index``. Using
composition here is better and reduces coupling.

This is a follow-up on the discussion that happened in
https://github.com/scala-ide/scala-search/pull/24
Merge pull request #27 from dotta/transient/index-job-manager-alternative-design

IndexJobManager does no longer inherit from Index