make the opt-out form work even if JavaScript is disabled

This makes the opt-out form work even if JavaScript is disabled. I've attached two screenshots, showing the current state and the new "JS is disabled, so we need a proper submit button" state, both using the default size of 600x200 pixels for the iframe.

**previous state with and without JavaScript + new state with JavaScript:**
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;![piwik-opt-out-a](https://cloud.githubusercontent.com/assets/800119/3605094/abc25e00-0d27-11e4-9baf-013a24fadecc.png)

**new state without JavaScript:**
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;![piwik-opt-out-b](https://cloud.githubusercontent.com/assets/800119/3605095/abc42028-0d27-11e4-80ef-f39386de0b55.png)

The result is not that beautiful, but should still fit into the iframe in every language.

Let me know if the button text itself should be translatable as well, as the browser's language is just used currently.