Alerts more metro style

Alert components was transformed similar to metro tiles or W8
notifications, using flat design
Fixing text color in Alerts

Fixing white color for text in metro style Alerts
Update docs on .alert-link description
Using LESS variables to set background colors

Preserving theme colors using LESS variables to set Alert background
colos the same as Buttons component
Merge pull request #53 from alexbruno/master

Alerts more metro style