Prefix C object names with git2r_ in src/git2r_blob.c/h
Added test case to check size of blob

* tests/blob.R: Added test case to check size in bytes of blob.
Prefix C object names with git2r_ in src/git2r_checkout.c/h
Added more checkout test cases

* tests/checkout.R: Added test case to checkout tag. Added tests to
  check that checkout gives expected result when reading file.
Fix checkout tag

* src/git2r_checkout.c (git2r_checkout_tag): Fixed implementation of
  checkout of S4 class git_tag target
Prefix C object names with git2r_ in src/git2r_clone.c/h
Roxygenized
Diff support (incomplete, to be overwritten)