Prefix C object names with git2r_ in src/git2r_commit.c/h
Added target Makevars in Makefile

* Makefile: Added target Makevars to rebuild Makevars.in and
  Makevars.win
Fix missing prefix git2r_ in .Call

* Added prefix in the call to:
  - git2r_commit
  - git2r_descendant_of
  - git2r_parents
Fix tree pointed to by a commit or stash

* R/tree.r (tree): Added prefix git2r_ in .Call to function
  git2r_commit_tree

* src/git2r.c: Added missing git2r_commit_tree to callMethods
Prefix C object names with git2r_ in src/git2r_config.c/h
Fix broken build
Prefix C object names with git2r_ in src/git2r_error.c/h
Prefix C object names with git2r_ in src/git2r_push.c/h
Prefix C object names with git2r_ in src/git2r_repository.c/h

* src/git2r_repository.c/h: Changed function names to have similar
  naming scheme as libgit2 with function names also prefixed with
  modulename. git2r_modulename_functionname() (e.g. git2r_repository_open())
Prefix C object names with git2r_ in src/git2r_repository.c/h
Prefix C object names with git2r_ in src/git2r_stash.c/h
Roxygenized
Update libgit2 to development branch
Add remotes support (will be overwritten)