List blobs from the commits in the object database

* src/git2r_odb.c/h (git2r_odb_blobs): List blobs reachable from the
  commits in the object database. First list all commits, then iterate
  over each blob from the tree and sub-trees of each commit. Return
  list with blob information vectors. The list contains the following items:
  - sha: the sha of the blob
  - author: the author of the commit
  - when: the timestamp of the author signature in the commit
  - path: the path to the blob from the tree and sub-trees
  - name: the name of the blob from the tree that contains the blob
  - len: the length of the blob
  - commit: the sha of the commit

* Note: A blob sha can have several entries

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