feat tabs: add color themes

Indicator, active bar and ripple colors can be changed.
chore demo : add color themes on tabs + fix color

Colors themes example on tabs feature and fix colors for link examples.
feat tabs: add scope variables and dark/light theme

Colors are now defined with scope variables. Colors managed with a
light an dark theme, following google specifications :
http://www.google.com/design/spec/components/tabs.html
chore demo: tabs demo updated with themes and colors
feat tabs: change default theme and parameter name
chore demo: tabs demo updated with renamed parameter and theme
chore demo: add a background color to a tab demo
Merge pull request #14 from lumapps/feat/tabs

feat tabs: add color themes