A few improvements and big fixes to allow the plugin to work with IE and FF

1. Introduction of `undefined` parameter to allow direct comparison
2. Nicety: a use of `$.proxy()` to eliminate use of the `that` var
3. `_Axis` is supplied with the `var` keyword (to prevent global scope
pollution)
4. `.convertColor()` function rewritten to eliminate use of `eval()` AND
consider `transparent` value of a color
5. If initial bg-color is `transparent` alpha is set to `0`, and color -
to `#ffffff`
6. Color calculation in init goes without eval now (there's no `eval()`
now in Flippy)