Fix for IE's 4095 selector/rule limit

Versions of IE prior to 10 had a relatively short 4095 selector/rule limit which may inadvertently be hit as a result of concatenating all the layers CSS documents together. 

http://support.microsoft.com/kb/262161

Added option to disable concatenation via RJS config using the "concatCSS" flag and setting to "false". The default remains to concatenate unless this option is explicitly set to false. This places the burden on the developer to ensure limits are not reached rather than to tool inadvertently introducing an error. 

During build optimizations, since the stylesheets are (generally) inlined anyways, it really shouldn't make any difference to performance. Rather than 1 inline style block per module/layer, you end up with 1 inline style block per CSS document.