Keep reference to `this`

Hi. I'm working on semi-conservative refactoring of mixins in jade (all existing unit tests are passed, only private api slightly changed) to make them first class objects, as well as proper handling of `this` refernece in templates. In classical jade `this` reference is used for passing of {block,attributes} to the mixin like 
```javascript
    var buf, jade // runtime vars enclosed into template's closure 
    mixin.call({block,attributes}, args...)
```
I've refactored lexer/parser/compiler + with
 to make mixins essentialy a curried functions while decoupling them from jade-runtime and buf:
```javascript
      mixin(args...)(attributes,block)(buf, jade)
```
so now mixins can be compiled separately with `jade.compileMethod(src, options)` and passed as parameters for templates, or attached to function prototype to make templates as methods, or simply bound by `mixin.bind(someobj)`

also i've added first-class mixin call syntax
```jade
       +[expression_that_evaluates_to_mixin]#id.class(data-test=123)(arg1,agr2,arg3)
             span I'm in block
       +[jade_mixins["test"]](1,2,3)
       +test(1,2,3)
       +[obj.renderView] //- this was a primary goal
```

All this features are based on small enhancement of `with` module to make it respectful for `this` reference.
My semantics/syntax for mixins is arguable but I hope that at least this change in `with` matters and can be merged.

Regards, Anatoly
P.S. my fork of jade is here https://github.com/Artazor/jade - it would be great to hear any comments about that direction
Stop!!! Then you should change AST checking too! otherwise it will fail.