Add animation to sweep progress bars with bootstrap class. Resolves #77.

Summary of changes:
 * Changes #sweep_progress_bar view helper by adding different classes
 to element depending on the number of pending sweeps.
 If pending sweeps are 0, (and the sweep is completed), then the class
 doesn't have animated stripes.
 * Adds 2 new specs to check that class is being appropriately applied
 depending on the number of pending sweeps.
 * All specs passing.

Reason:
 * Striped animation makes progress bar more obvious and engaging.
 * Change from stripes to solid signals visually the sweep completion.

Drawbacks:
 * New specs for this feature rely on existing HTML structure somewhat.
 * Specs for this feature may be more appropriate as feature specs;
 added to existing sweeps_helper_spec.rb since the current specs seem to
 focus on unit testing.