Add method to create blob from a file

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

* src/git2r_blob.c/h (git2r_blob_create_fromdisk): Added C code to
  to create the blob from file.

* src/git2r.c: Added "git2r_blob_create_fromisk" to callMethods

* man/create_blob_fromdisk-methods.Rd: Added documentation

* NAMESPACE: Updated

* tests/blob.R: Added test cases