Basic support for I18n contexts as requested by #2063

I've tried creating the basics for this. It can only read the translations using contexts (not extract them yet) and it only has the ```__x``` function for now (as proposed by @garas in issue #2063).
Please break in two lines:

````
if (($trans = $_this->_domains[$domain][$_this->_lang][$_this->category][$singular][$context]) ||
	($plurals) && ($trans = $_this->_domains[$domain][$_this->_lang][$_this->category][$plural][$context])) {
````