Pluralize Project API

This changes all APIs concerning paths and repositories on the project
to be plural, preparing us to switch to multi-folder projects. It
doesn’t make any changes to actually support multiple folders. Instead
we just wrap the previous return values in singleton arrays.

* constructor ‘path’ params -> ‘paths’
* getRootDirectory -> getDirectories
* getPath -> getPaths
* setPath -> setPaths
* getRepo -> getRepositories

src/text-editor.coffee

@@ -133,8 +133,8 @@ class TextEditor extends Model
   subscribeToBuffer: ->
     @buffer.retain()
     @subscribe @buffer.onDidChangePath =>
-      unless atom.project.getPath()?
-        atom.project.setPath(path.dirname(@getPath()))
+      unless atom.project.getPaths()[0]?
+        atom.project.setPaths([path.dirname(@getPath())])
       @emit "title-changed"
       @emitter.emit 'did-change-title', @getTitle()
       @emit "path-changed"