Fixes gauge-color attribute to parse as documented

`gaugeColor`, like other color options, is now bound as a local scope property rather than a bi-directional binding that previously required a quoted value like `gauge-color="'#ff0000'"`. I would consider this expected behavior since the other colors are defined that way and the project documentation suggests simply `gauge-color="#ff0000"`, which now works.

Before the pull request, `gauge-color="#ff0000"` broke the directive due to an Angular parse error. Now, `gauge-color="#ff0000"` works. In case anyone used the `gauge-color="'#ff0000'"` syntax, this pull request includes backwards compatibility with that syntax by removing leading and trailing single quotes from the string value.

The "Custom" gauge in the demo has been updated with a custom gauge-color.