Add mixin support (except mixin.attrs & mixin.merge)

I tried to keep as close as possible to the jade implementation ;

 * removed all the indentation
 * kept the dynamicMixins stuff
 * added the `tags` array logic / concat for mixin calls & mixin blocks

all tests are passing except for the mixin.attrs & mixin.merge tests. I did not look into these since they are more related to the attrs handling and I am not sure about the status of this in react-jade ; maybe you will see things clearer than me on those).

Tell me if it looks ok to you !