More complete per project settings

This improves https://github.com/maltize/sublime-text-2-ruby-tests/pull/239 by:
- adding a `RubyTest` namespace to the per-project settings (see example below)
- allowing more RubyTest settings to be overridden per project, e.g. `check_for_spring`

### Example:

To ensure spring is used for this project regardless of what's in the user or default setting:
```json
{
	"folders": [],
	"settings": {
		"RubyTest": {
			"check_for_spring": true
		}
	}
}
```