Add support for theming

[Chris Kiehl]

No description provided.

[Iván Loire]

The first and basic approach is to have a themes folder, with custom themes on it (a folder per theme - i.e: "themes/darkblue". Current theme is defined in the configuration file.

We can have the default theme in atlasboard, and of course the user can drop their own themes on its concrete board project "mydashboard".

The theme manage should be able to resolve the actual theme name looking in both <atlasboard>/theme and <mydashboard>/theme folders

Ideally each dashboard file can have refer to its own theme, so different dashboards can have different themes within the same atlasboard project.

[Iván Loire]

In 1.0 widgets can use stylus and have access to the default Atlasboard color palette stylus variables which was the first step towards theming.

[Iván Loire]

This is done in pre-1.0-release and will be released in 1.0