Enabling cache directory

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/aqr@49 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
First stab at load and save xts from disc

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/aqr@50 edb9625f-4e0d-4859-8d74-9fd3b1da38cb