SourceMap: Decodings factory added

One step further to accomplish https://github.com/madskristensen/WebEssentials2013/issues/783.

Next steps:

- [ ] Cache the processed `CssSourceMap` instances using `DependencyGraph`, when map file is generated.
- [ ] Wire up selectivity logic with `CssSourceMap`.
- [ ] Add scrolling event handler to preview window for LESS and SCSS using source map (#783). This should preferably be smooth scrolling (if possible).
- [ ] Add context menu to preview window (#783).