Add support for current() function

[LukaszWiktor]

First of all, thank you so much for developing the xpath module for nodejs. It's working really well!

The only missing part I've just discovered is the standard XPath current() function. The current() function is indispensable in inner exprssions (e.g. in square brackets) where the context node . is different than the current node. Could you please add suppor for the current() function?