Initial work with reflog support
Fix Collate field in DESCRIPTION
Add C function git2r_reflog_list

* src/git2r_reflog.c/h (git2r_reflog_list): List reflog entries in
  specified reference

* src/git2r.c: Add git2r_reflog_list to callMethods
Add S4 class git_reflog_entry
Add show method for S4 class git_reflog_entry
Add reflog method to list and view reflog entries
Merge branch 'add-reflog-support'