named: Support of zones which are maintained in a git repository.

Some communities maintain there local zone file via git. These changes
make it possible to clone the repo and include the zone file into
the named configuration. This also includes a git hook and a update
script.

The script loops over such repository clones and run 'git pull'. When
changes are pulled in a hook will trigger named to reload its
configuration.
readme: Describe new type ffnord::named::zone
named: Exclude localy mastered zone.
Merge pull request #22 from sargon/named-zone-git

Integrate zone file from git repo.