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
You do not need to dereference pointers in Go `w.FileWatcher.Errors` will work fine.