[VarDumper] Use symfony.com's colorscheme

As promised, this PR changed the current colorscheme to something used in Symfony. This means developers have a consistent experience and the colorscheme is easier to read.

While doings this, I also improved the `HtmlDumper#style()` method to make it easier to understand and maintain. I also changed a bit of the output it created, mostly adding visibility prefixes to properties.

**Before**
![sf-dump-before](https://cloud.githubusercontent.com/assets/749025/4828241/b070d32e-5f7e-11e4-9d51-865ffd47753e.png)

**After**
![sf-dump-after](https://cloud.githubusercontent.com/assets/749025/4828244/b4ec3e52-5f7e-11e4-9031-71f41bc30c25.png)


| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | -
| Deprecations? | -
| Tests pass?   | -
| Fixed tickets | #12348 
| License       | MIT
| Doc PR        | the one about the VarDumper