prefixes variables and makes them overridable

The current implementation of the less stylesheets makes it impossible to override the variables from the users own less file because the variables are scoped.
You had to manually copy the code in the less file and change the variables there.
This PR moves the variables out of the scope and prefixes them so they don't interfere with other variables that may exist in the user's code.

you now can do the following to override the colors:

```less
//my-own-styling.less
@import "path/to/dist/angular-wizard"; //import the file

@wz-color-default: #EEE; //override the variables
@wz-color-done: #0000FF;
```