feat(linear-progress): Add linear progress indicator

This linear progress directive displays a simple, animated loading bar. There are four display modes: determinate, indeterminate, buffer and query.

Closes #187
We should add ARIA stuff here. 

`element.attr({
    'role' : 'progressbar',
    'aria-valuemin' : 0,
    'aria-valuemax' : 100
});`
`element.attr('aria-valuenow', someValueThatGetsUpdated);`