Fix IE css sheet limitation

This will fix the limitation in IE of the number of CSS style sheets allowed (<IE10?) which is 31 sheets.  When using SlickGrid to pragmatically create grids on a page with IE each grid adds a CSS style sheet.  I've seen where this hits the max 31 fairly easily.  This simple change will have SlickGrid append to one single CSS sheet when using IE rather than adding a new one for each grid.