Scss files support

[xogg]

Content Filters
Adds a new abstract file type BT.CodeFile with content filtering support implemented via a BT.ContentFiltersMixin.
With content filtering mixed in, a chain of filters may be defined for a processable file with purpose of parsing, replacing known fragments or any other kind of processing.

sc_static() / static_url() support
Implemented as JavaScript functions in node-sass.
all targets are inserted into the file's resourceDependencies, so linked resources would be processed in BUILD run mode.

Dependency tree
All @import's in .scss files are intercepted and a tree of dependencies is maintained.
Updating an imported file leads to the update of all master files on top of its dependency chain.

[mauritslamers]

I like the idea a lot, because I see its value for the script files as well as base class for scripts and styles of any kind. I am unsure about this implementation.
First of all, the class name is a bit vague, BT.FilteredFile would be better IMHO.
Secondly, I think that this doesn't need to be done through mixins, mainly because if the normal extending procedure is used, you can use concatenatedProperties (http://docs.sproutcore.com/#doc=SC.Object&method=concatenatedProperties&src=false) to automatically attach new filtering to the end of the filtering procedure.
Moreover, if the filters themselves are implemented as cached computed properties, it could mean a speed up for any processed file.

The purpose of the class itself is to simply apply all the filters in a row.
Thoughts?

[nicolasbadia]

Very nice work! I agreed with Maurits for the implementation details.

[nicolasbadia]

I thinks we should also move most of the BT.CSSFile logic into BT.SCSSFile, and extend BT.CSSFile from BT.SCSSFile.