List all objects available in the database

* R/obd.r (odb_list): List all objects available in the database and
  return a list with S4 class git_object's.

* src/git2r_odb.c/h (git2r_odb_list): Internal function to iterate
  over all objects and create git_object's.

* src/git2r.c: Add git2r_odb_list to callMethods.

* man/odb_list-methods.Rd: Add documentation.

* DESCRIPTION: Add odb.r to collate field

* NAMESPACE: Export method odb_list

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>