fixes #78; added numeric convertion to getValueBinding

Added conversion to number when lvalue's type is number in getValueBinding which seems to fix #78
Also removed usage of unitialized variables "val" and "newVal" and replaced them with "$el.val()" which fixes 'Variants and Pattern Matching' sample throwing "ReferenceError: newVal is not defined" amongst other things.

One thing to note is that I have no idea if recalculation of "+$el.val()" is a performance concern here:
``` javascript
eval(nweb.utils.makeAssignExpression(expr, "+$el.val() ? +$el.val() : $el.val()"));
```