Refactor a bit the library + add some public methods

It's quite a big pull request I propose here: Circles library is awesome but it definitively needs some customization :)
I wanted to go further than simply add a method `updatePercent()` (see #11), so this pull request changes some things:

* we don't use anymore properties `percent` and `number`: we can now simply provide a `value` and a `maxValue` and the library calculates the percentage by itselves.
* `text` can be anything we want: an empty string, a static string, an HTML string or even a function, so that we can custom the center of the circle as we want to.
* old `generate(radius)` is now `updateRadius(radius)`
* a collection of new methods are now public: `update(Number number[, Number duration])`, `update(Boolean force)`, `updateWidth(Number width)`, `updateColors(Array colors)`, `getPercent()`, `getValueFromPercent(Number percent)`, `htmlifyNumber(NUmber number[, String integerPartClass, String decimalPartClass])`

You can see this patch working here: http://adrien-gueret.github.io/#circles

Please let me know if all is OK for you :)