Allows settings to be overwritten on a per-project base

If you create a `settings` area on your `Project.sublime-project` file and you override settings of the original `RubyTest.sublime-settings` file, it will read the properties from that file that instead.

Example (used on a Rails 3.2 project that uses Spring binstubs for RSpec):

```javascript
{
	"folders":
	[
		{
			"follow_symlinks": true,
			"path": "crm_bliss"
		}
	],
	"settings":
	{
		"run_rspec_command": "bundle exec bin/rspec {relative_path}",
		"run_single_rspec_command": "bundle exec bin/rspec {relative_path}:{line_number}",
	}
}
```