Refactor create blob methods

* R/blob.r (blob_create): Merge the two methods 'create_blob_fromdisk'
  and 'create_blob_fromworkdir' to the method 'blob_create'. Add
  argument 'relative' to determine if the files are relative to the
  repository's working directory. Default is TRUE. Add example.

* tests/blob.R: Update tests.

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