tracker should depend on a miner

[PeterPall]

It is possible to install tracker without the package tracker-miner-fs. But without this package tracker is completely helpless and even unable to produce a meaningful error message:

tracker-control -S

will crash in this case, the gui tools won't tell that anything is wrong.

Why this is bad:
 - If on a minimal system the "gnome" virtual package is installed tracker is automatically installed without a tracker.

Notes:
 - A google and a package search didn't give a hint that any other tracker than tracker-miner-fs currently exists. We should be prepared for new miners to appear, though.

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: tracker 0.16.1-1ubuntu2
ProcVersionSignature: Ubuntu 3.11.0-3.7-generic 3.11.0-rc6
Uname: Linux 3.11.0-3-generic x86_64
ApportVersion: 2.12.1-0ubuntu2
Architecture: amd64
Date: Sat Aug 24 09:00:22 2013
EcryptfsInUse: Yes
MarkForUpload: True
SourcePackage: tracker
UpgradeStatus: No upgrade log present (probably fresh install)