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
Added the default case.  We also only rebuild on fsnotify.Write.  Should we rebuild on Rename, Remove, and Create?  My thoughts are only Rename.