Some improvement related to id.col

* Use identical(id.col, character(0)) to detect character(0)

* Add .id.col to db.Rquery properties

* When creating db.Rquery object, add .id.col together with the column
  extracted if .id.col is not character(0)
Expose the string to array conversion utility to user

* Changed the name from .str2vec to arraydb.to.arrayr

* Added Rd file

* For a single db array, returns a vector