Added restat functionality to be compatible with FileMQ

* The previous behaviour was to automatically restat on every accessor
  method, which is simple, but makes it impossible to compare old and
  new states by using two zfile objects.

* Accessor methods now do not call zfile_restat() any longer. Caller
  must do this explicitly when needed.

* Updated test case for this.
Merge pull request #257 from hintjens/master

Added restat functionality to be compatible with FileMQ