add rake target for coverage report
add rake target for coverage report

mrtazz/plustache,thecocce/plustache,kylealanhale/plustache,chesles/plustache,cgcgbcbc/plustache,cgcgbcbc/plustache,ShiweiJuliaHuang/plustache,kylealanhale/plustache,ShiweiJuliaHuang/plustache,thecocce/plustache,chesles/plustache,mrtazz/plustache,thecocce/plustache,kylealanhale/plustache,ShiweiJuliaHuang/plustache,cgcgbcbc/plustache,chesles/plustache