Rename and refactor odb_list method

* R/odb.r: Rename method odb_list to odb_objects. Refactor to return a
  data.frame with the columns; sha, type and len instead of a list
  with S4 class git_objects.

* src/git2r_odb.c/h: Rename git2r_odb_list to git2r_odb_objects.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Remove unused S4 class git_object
Roxygenize

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add test for odb_objects method

* tests/tag.R: Add test case for odb_objects method. Calls C function
  git2r_odb_objects.

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