add basic GitAccess class

lib/gollum/git_access.rb

@@ -0,0 +1,19 @@
+module Gollum
+  class GitAccess
+    attr_reader :path
+    attr_reader :repo
+    attr_reader :ref_map
+    attr_reader :tree_map
+
+    def initialize(path)
+      @path     = path
+      @repo     = Grit::Repo.new(path)
+      @ref_map  = {}
+      @tree_map = {}
+    end
+
+    def exist?
+      @repo.git.exist?
+    end
+  end
+end