Added hashfile method to determine the sha1 hex of a blob from file

* R/blob.r (hashfile): Added generic hashfile and method for signature
  character to determine the sha1 hex of a blob from a file

* src/git2r_odb.c/h (git2r_odb_hashfile): Added C code to determine
  hash from file. The blob is not written to the object database.

* src/git2r.c: Added "git2r_odb_hashfile" to callMethods

* man/hashfile-methods.Rd: Added documentation

* NAMESPACE: Updated

* tests/blob.R: Added test cases