Added slot path to S4 class git_commit

* R/commit.r: Added slot path to hold the path to the git repository
  that contains the commit
Fixed git2r C code to init S4 class git_commit path slot
Added method to get tree pointed to by a commit

* R/tree.r (tree): Added generic tree and method for S4 class
  git_commit to get the tree pointed to by a commit.

* man/tree-methods.Rd: Added documentation

* Added C function commit_tree to get tree from commit

* Updated NAMESPACE
Added tree method test case
Fix failing tree test case

* src/git2r_commit.c (commit_tree): Fix code, the tree was erroneously
  initiated from the commit and not the tree object.