Added hex slot to S4 class git_tag
Added C code to init a S4 class git_tree

* src/git2r_tree.h: Added

* src/git2r_tree.c: Added
Updated Makevars
Improved lookup method to also handle commit, tag and tree

* git2r_repository.h/c (lookup): Moved code from git2r_blob.h/c. The
  lookup function now also handles commit, tag and tree data
  structures.
Minor change of function argument names

* src/git2r_commit.h/c (init_commit): Renamed arguments to source and
  dest to be consistent with the other init S4 class functions.
Added more lookup tests
Fix R CMD check issues