Add HTTP "Cache-Control" headers

[fredrikeldh]

Some clients (for example, Android 4.4), caches files such that they don't get reloaded when the workbench sends the "reload" signal.

It was suggested that the problem be remedied by "meta http-equiv no-cache" tags, but those must be added manually to each HTML file, and will not work for .js, .css or other files.

Instead, the HyperReload webserver might be modified to include the header "Cache-Control: must-revalidate" or "no-cache" in all responses.

This may increase load times if unmodified files are retransmitted, but in case of broken HTTP clients, this will be the only method to ensure consistent operation.