Two Bug Fixes

This pull request addresses two issues affecting IE:

1. When attaching multiple stylesheets using the css plugin, only the last sheet will be applied in IE <= 10 as each sheet clobbers the sheet before it. The problem is using innerHTML on dynamically created style elements (which is what the plugin does for browsers--basically IE--identified by feature sniffing the styleSheet object on style elements). Mainly, innerHTML tends to return an empty string regardless of the content, so when multiple stylesheets are attached only the last one is available (as each preceding sheet clobbers one another). The provided solution is to cache the stylesheet content for subsequent fetches can concat.

Also, after discussing with @eoneill, a few more updates in a future patch will guard against other issues involving charset and media declarations, and overall selector limit.

2. The removeGlobalObject method in executor deletes a window property, which IE8 doesn't like. I've added a try/catch fallback which is setting the property to undefined. This makes IE8 happy.

I'm getting this error in Travis:
https://travis-ci.org/gvsboy/inject/builds/31279503
Do you know what I need to do to remedy? Some configuration I'm missing?