Adding combine option

Dear Gabriel,

Please consider this PR fully dedicated to the inclusion of combine option.

With this PR, when debug => true and combine => false, then the css and js are not merged but left as is to ease the development work.

There is one additional thing which could be done : If you switch from combine => true to combine => false, as the js and css have not changed, these are not written to disk. You have to modify one of the files included in the collection to make them generated, which is not that good. So, as an improvement of this PR, we could trigger the rebuild of files in case of switch between combine=>true to combine=>false.

Let me know your feeling about that.

Greg