Fixed issues with old Chrome versions:

* There was an intermediate messaging API where messages were sent via chrome.extension.sendMessage/chrome.tabs.sendMessage and received with chrome.extension.onMessage
* Accessing (not calling) chrome.extension.getBackgroundPage used to cause an exception
* Messages received by content scripts got a fake sender tab with ID -1
* Pop-up should not assume always getting a response to its messages

Review: http://codereview.adblockplus.org/5867629902299136
Updated locales from Crowdin