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
Add Project::onDidChangePaths event
Merge pull request #3691 from atom/ns-pluralize-project-api

Pluralize Project API