Base all relative paths from the project path, not from cwd

[Dinnerbone]

I have a project in Sublime Text 3 that includes . as one of its folders. Snipped down, it looks like this:

{
    "folders":
    [
        {
            "path": ".",
            "name": "Project"
        }
    ]
}

With this project, PluginUtils.project_path() returns "." which is then expanded with cwd ("C:\Program Files\Sublime Text 3" for me) and fails to find eslint.

This pull request uses a base dir of the project itself, then fails back to cwd if there's no project open.

[TheSavior]

Thank you very much for making these separate PRs. Made it easy to review.