Added hash method to get the sha1 hex of a blob from a string

* R/blob.r (hash): Added method hash to determine sha1 hex of a blob
  from a string

* src/git2r_odb.c/h (git2r_odb_hash): Added C code to
  determine hash. The blob is not written to the object
  database.

* src/git2r.c: Added "git2r_odb_hash" to callMethods

* man/hash.Rd: Added documentation

* NAMESPACE: Updated

* tests/blob.R: Added test cases