delete file on delete(); add store FSDirectory

* on delete action - erase the acutal messagepack file used to build the
  index

* add store => path option index setting, that tells the Mapping class
  to actually store the index on the file system instead only in
  RAMDirectory().

  It checks if total number of documents in all the shards matches the
  expected number of documents in the index's settings. if not, it will
  erase the stored index and rebuild it from the messagepack file
  on delete() it will also erase the messagepack file used to build the index.

  + works only when building index from a messagepack file
add RAMBrick to the enabled bricks