Add method to create blob from a file in workdir

* R/blob.r (create_blob_fromdisk): Added generic
  create_blob_fromworkdir and method for signature git_repository and
  character to read a file in the workdir and write its content to the
  Object Database as a loose blob.

* src/git2r_blob.c/h (git2r_blob_create_fromworkdir): Added C code to
  to create the blob from file.

* src/git2r.c: Added "git2r_blob_create_fromworkdir" to callMethods

* man/create_blob_fromworkdir-methods.Rd: Added documentation

* NAMESPACE: Updated

* tests/blob.R: Added test cases