Filewatcher

Filewatcher represents a big refactor to wellington.  It introduces fsnotify events and reworking of parser/context relationship to support more flexible building scheme.  The following events trigger rebuilds.

    file.scss
    \--- _import1.scss
    \--- _import2.scss
    file2.scss
    \--- import.scss
    \--- import2.scss

Touching _import1.scss rebuilds file.scss, touching import.scss (despite bad name) updates file2.scss
This `done` channel is in the fsnotify example but is not needed for us, the routine will live for as long as the for loop continues.  It would be advantageous to add a default: case for nothing else than testing.

    select {
    default:
       // only called when w.FileWatcher.Events is set to nil.