Added method to get content of a blob

* R/blob.r (content): Added generic content and method for S4 class
  git_blob to get content. Return content either as a vector of
  character strings or as one character string

* man/contents-methods.Rd: Added documentation

* src/git2r_blob.c/h (git2r_blob_content): Added C code to get rawcontent
  of a blob.

* src/git2r.c: Added "git2r_blob_content" to callMethods

* NAMESPACE: Updated