Merge pull request #143 from pduranti/master

New guides for shipping with MercadoEnvios
Update _posts/guide/for-sellers/shipping/2013-01-08-shipping-overview.markdown
Update _posts/guide/for-sellers/shipping/2013-01-08-shipping-overview.markdown
Update _posts/guide/for-sellers/shipping/2013-01-08-shipping-overview.markdown
clarify details on ship me1 and me2